Part I - Treaty versus Custom

Introduction

Disagreements about the content of international law, particularly in the field of jus contra bellum, often begin due to differently held assumptions about the legitimate process for identifying the content of the law.¹ ‘Method, far from being a theoretical preoccupation, lays down the framework in which practice takes place.’² This part sets out the theoretical foundation and method for determining the meaning of a prohibited ‘use of force’ between States in international law. The prohibition of the use of force exists under two main sources of law: customary international law³ and treaty (article 2(4) of the UN Charter). It is practically axiomatic that the prohibition of the use of force has an identical scope and content under both article 2(4) of the UN Charter and customary international law. Already in 1966, Sir Humphrey Waldock observed: ‘Whatever may be their opinions about the state of the law prior to the establishment of the United Nations, the great majority of international lawyers consider that Article 2, paragraph 4, together with other provisions of the Charter, authoritatively declares the modern customary law regarding the threat or use of force.’⁴

If the scope and content of the prohibition of the use of force under article 2(4) and custom are identical, which source of law should one interpret or apply to determine the meaning of a prohibited ‘use of force’ between States under international law: article 2(4), customary international law, or both? This question raises several fundamental issues. Firstly, are the scope and content of the prohibition of the use of force under article 2(4) of the UN Charter and customary international law really identical? Secondly, is it even possible to adduce the content of the customary rule separately to the treaty rule? The novel contribution of this part is to analyse how the customary prohibition of the use of force arose, and its relationship to article 2(4) of the UN Charter. It argues that the customary rule reflects the pre-existing treaty rule and that due to the relationship between them, the preferable approach is to focus on interpreting the UN Charter to determine the meaning of a prohibited ‘use of force’ under international law.

1 How and When Did the Customary Prohibition of the Use of Force Emerge? The Status of the Customary Norm Pre-1945

Introduction

The question of whether the prohibition of the use of force is identical under the UN Charter and customary international law is fundamental to deciding the approach to take to discover the meaning of prohibited force under international law. If they differ in some way, then it would be necessary to adduce the content under each source separately. Even if the customary and treaty prohibitions of the use of force are presently identical in scope and content, the current relationship between the two is especially relevant to potential future changes in the prohibition under both treaty and custom,¹ as we shall see later in Chapter 3. In particular, there are significant differences in the way that the rule may evolve through subsequent practice in the application of the treaty versus evolution of custom, as well as limits to such changes including the constraints of informal treaty modification and the peremptory nature of the prohibition. For these reasons, it is essential to commence our enquiry by examining the relationship between the treaty (UN Charter) and customary prohibitions of the use of force: are they indeed identical, what is their present relationship, and which should we interpret or apply to discover the meaning of prohibited force under international law? The starting point for this enquiry is the origin of the customary rule: how and when did it actually emerge, and what is its relationship to article 2(4) of the UN Charter?

The Nicaragua Case

Before we continue, let us first address and dispense with the case that is often proffered as the answer to these questions: the Nicaragua case. Certainly, the International Court of Justice (ICJ) in the Nicaragua case affirmed that there is a customary prohibition of the use of force.² However, as we shall see, the Court did not actually hold that the content of the customary prohibition is identical to the prohibition in article 2(4), and its assertion of how and when the customary norm emerged is problematic.

In the Nicaragua case, the ICJ found that it had jurisdiction to determine the dispute on the basis of customary international law only, and not the UN Charter due to the US reservation to the Court’s jurisdiction.³ In its judgment on the merits, the Court indicated its view that the principles of the non-use of force and of the right to self-defence were already present in customary international law before the Charter and that these parallel (and largely identical) customary rules ‘developed under the influence of the Charter’. The Court held:

[S]o far from having constituted a marked departure from a customary international law which still exists unmodified, the Charter gave expression in this field to principles already present in customary international law, and that law has in the subsequent four decades developed under the influence of the Charter, to such an extent that a number of rules contained in the Charter have acquired a status independent of it. The essential consideration is that both the Charter and the customary international law flow from a common fundamental principle outlawing the use of force in international relations.⁴

However, the ICJ did not explicitly hold that the prohibition under each source of law was identical, and its analysis in identifying the parallel customary rule has been rightly criticised. The Court was rather obtuse about whether the prohibition of the use of force in article 2(4) is exactly the same in customary international law. It stated:

The Court does not consider that, in the areas of law relevant to the present dispute, it can be claimed that all the customary rules which may be invoked have a content exactly identical to that of the rules contained in the treaties which cannot be applied by virtue of the United States reservation. On a number of points, the areas governed by the two sources of law do not exactly overlap, and the substantive rules in which they are framed are not identical in content.⁵

The Court re-states this point in the following paragraph, holding that ‘[t]he areas governed by the two sources of law thus do not overlap exactly, and the rules do not have the same content’.⁶ Claus Kreß argues that despite the ICJ’s statements, subsequent parts of the judgment show that it has interpreted customary international law and article 2(4) ‘in a largely identical manner’.⁷ Furthermore, since in the Armed Activities case, the ICJ referred to the ‘principle’ of the non-use of force in international relations without citing its source,⁸ Kreß concludes that it is based on ‘essentially identical rules of treaty and customary law existing alongside each other’.⁹ However, this finding was far from explicit, and other scholars have noted that the ICJ seems to treat the two as identical in substance without much analysis.¹⁰

The ICJ has also been criticised for its reasoning in identifying the parallel customary prohibition of the use of force. Despite its frequent references to the need to evaluate the existence of a general practice accepted as law in order to identify a rule of customary international law and its holding that ‘[t]he Court must satisfy itself that the existence of the rule in the opinio juris of States is confirmed by practice’,¹¹ Christine Gray notes that ‘[the Court] was criticized for inferring opinio juris from General Assembly resolutions and for not undertaking a wide survey of practice’.¹² The Court also failed to clearly distinguish between practice in the application of the treaty and State practice and opinio juris under customary international law. It noted that Nicaragua and the USA ‘accept a treaty-law obligation to refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations’.¹³ The Court correctly held that it ‘has however to be satisfied that there exists in customary international law an opinio juris as to the binding character of such abstention’.¹⁴ Oscar Schachter observes that ‘[j]ust how the Court could tell whether practice since 1945 by the treaty parties relative to the use of force was “customary” rather than treaty is not made clear.’¹⁵ The Court also relied on multilateral conventions such as the UN Charter and the Charter of the Organization of American States to ascertain the content of the customary rule without further explanation.¹⁶

These deficiencies in the judgment and the fact that the Court left open whether the customary and UN Charter prohibitions of the use of force are actually identical mean that the Nicaragua case is not the end of the road in our quest to discover whether the prohibition is identical under each source of law, and their present relationship. The rest of Part I will examine this question afresh.

How and When Did the Customary Prohibition of the Use of Force Emerge?

There are four possibilities for how and when the current customary prohibition of the use of force between States arose.¹⁷ The first possibility is that the customary rule developed prior to the UN Charter and that article 2(4) was declaratory of that pre-existing custom. The second possibility is that article 2(4) crystallised a rule of customary international law that was by 1945 already in the process of formation. The third possibility is that article 2(4) gave rise to a new rule of customary international law in the usual way, that is, through subsequent State practice and opinio juris (the two-element approach). The fourth possibility is that article 2(4) gave rise to a new customary rule from its own impact, due to its ‘fundamentally norm-creating character’ ‘accepted as such by the opinio juris’ and a sufficient number of ratifications and accessions to imply a ‘positive acceptance of its principles’ and ‘extensive and virtually uniform’ State practice.¹⁸ The following discussion will canvass the first of these two possibilities and examine the status of the customary norm prior to 1945. Chapter 2 will then focus on the status of the customary norm in the UN Charter era and whether it is currently identical to the rule in article 2(4) of the Charter.

The Status of the Customary Norm Pre-1945

Article 2(4) as Declaratory of Pre-existing Customary International Law?

The first possibility is that article 2(4) was declaratory of a customary international law rule prohibiting the use of force between States that pre-dated the 1945 UN Charter. If article 2(4) was merely declaratory of such a customary rule, then the customary rule would continue to be in force alongside the Charter. For a pre-existing rule of customary international law prohibiting the use of force in the same terms as article 2(4) to have arisen prior to 1945, the requirements of a general practice accepted as law must have been present prior to that date. This was not the case. Rather, article 2(4) of the UN Charter was a significant new legal development.

Pre-Charter Era

Prior to 1945, there were legal developments restricting the right to resort to war between States, but this fell short of outlawing ‘use of force’. The historical trajectory of the prohibition of the use of force has, broadly speaking, traced a liberal attitude towards war, in which rulers were absolutely free to resort to war, to the development of a moral discourse on war in the form of just war theory, which gave an account of the conditions under which resort to war was righteous.¹⁹ Just war doctrine has its roots in Roman law and the early writings of Saint Augustine, and came to fruition during the Middle Ages.²⁰ Prior to the twentieth century, there was no international legal regulation of the use of force between States.²¹ The Hague Peace Conferences of 1899 and 1907 were the first attempts to restrict such freedom to resort to force and included modest restrictions.²²

During the inter-war period (November 1918 to September 1939), efforts to restrict legal resort to war between States intensified. The two most notable international instruments during this period were the Covenant of the League of Nations,²³ and the 1928 Kellogg–Briand Pact (General Treaty for Renunciation of War as an Instrument of National Policy).²⁴ The Covenant of the League of Nations required peaceful dispute settlement between States and provided for a system of collective security and sanctions.²⁵ The League Covenant of 1919 contained exceptional qualifications on the right to resort to war. ‘Resort to war in violation of the Covenant was illegal but the content of the illegality was prima facie the violation of a treaty obligation.’²⁶ However, the Covenant did not prohibit war if dispute settlement was unsuccessful, after a cooling-off period, and ‘it did not restrict use of force other than war and aggression’.²⁷ From 1919, there were a number of international instruments variously declaring aggressive war/wars of aggression as an international crime (e.g. the Draft Treaty of Mutual Assistance, which never entered into force; the 1925 Sixth Assembly resolution: ‘war of aggression’ is ‘an international crime’; the 1927 Eighth Assembly resolution: ‘wars of aggression are … prohibited’). But this ‘just affirmed existing international law’ and ‘did not go beyond the [League] Covenant’.²⁸ The 1928 Resolution of the Sixth International Conference of American States also considered and resolved that aggression is ‘illicit and as such declared as prohibited’, but there remained the problem of a lack of definition.

The turning point which galvanised the emerging international law prohibiting recourse to war was the 1928 Kellogg–Briand Pact: the General Treaty for Renunciation of War. The parties to the Pact ‘condemne[d] recourse to war for the solution of international controversies, and renounce[d] it, as an instrument of national policy in their relations with one another’.²⁹ ‘[W]ar in violation of the Paris Pact was equated to aggression, triggering the obligations of third states under Article 10 of the Covenant.’³⁰ The Pact did not provide for sanctions, though violation did have consequences, for example, liability for damages, a right of intervention and no rights arising from a war in violation of the Pact.³¹ Ian Brownlie notes, ‘[t]he treaty was of almost universal obligation since only four states in international society as it existed before the Second World War were not bound by its provisions’.³²

It is controversial whether these legal developments amounted to the creation of a customary rule prohibiting force that was merely replicated later in article 2(4) of the UN Charter. Brownlie took the position that these multilateral treaties – together with a multitude of bilateral treaties during this time period reflecting similar provisions, various statements by States demonstrating an acceptance of the legal nature of the obligation to refrain from recourse to force in international relations (though it seems that these statements really emphasise that the legal obligation stems from the Pact and the League Covenant) and State practice – support the conclusion that at least by 1939, resort to war was illegal unless in self-defence.³³ However, he acknowledges that ‘[t]here was no general agreement on the precise meaning of the terms used in instruments and diplomatic practice relating to the use of force. This still creates serious difficulty but it is absurd to suggest that because there is a certain degree of controversy the basic obligation does not apply to the more obvious instances of illegality.’³⁴

Many of the legal developments referred to earlier in the chapter did not explicitly prohibit ‘force’, but ‘war’, which may have been a broader term. ‘Whether “war” in the Pact was used in its technical meaning and all other uses of force were excluded was and remains a matter of contention among international lawyers.’³⁵ Brownlie argues that ‘[t]he subsequent practice of parties to the Kellogg-Briand Pact leaves little room for doubt that it was understood to prohibit any substantial use of armed force’.³⁶ Randall Lesaffer believes that Brownlie’s view is too ‘rosy’ a picture, since State practice post-World War II ‘indicates that states still considered themselves to have a right to resort to war and formally declare war in the case of prior aggression by an enemy. Moreover, the Covenant and the [Kellogg–Briand] Pact had left the door wide open for an alternative strategy to resort to force rather than war, primarily in the guise of self-defence.’³⁷

The UN Charter Era

After the conclusion of World War II, a new era of international law was ushered in with the advent of the UN Charter in 1945, and, in particular, its cornerstone provision in article 2(4) prohibiting the ‘use of force’ between States. As Hans Kelsen notes, ‘[t]he Charter of the United Nations goes much farther than its predecessors. It obligates the Members of the United Nations not only not to resort to war against each other but to refrain from the threat or use of force and to settle their disputes by peaceful means (Article 2, paragraphs 3 and 4).’³⁸ The prohibition of a ‘use of force’ in article 2(4) was therefore a significant legal development in comparison to earlier international law existing at that time, which prohibited resort to ‘war’.³⁹

This view is also supported by statements made during the drafting of the Vienna Convention on the Law of Treaties, with respect to draft article 36. The draft article, entitled ‘coercion of a State by the threat or use of force’, provided that ‘[a] treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of the Charter of the United Nations.’ In the discussion of the draft provision, the Netherlands and the United States raised the question of its retroactive applicability. The United States noted that:

The traditional doctrine prior to the League Covenant was that the validity of a treaty was not affected by the fact that it had been entered into under the threat or use of force. With the Covenant and the Pact of Paris, this traditional doctrine came under attack; with the Charter it was overturned. In the view of the United States Government, it was therefore only with the coming into effect of the Charter that the concept of the illegitimacy of threats or uses of force in violation of the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations, was accepted.⁴⁰

This view was affirmed by Sir Humphrey Waldock and cited by Judge Jennings in the Nicaragua case: ‘The illegality of recourse to armed reprisals or other forms of armed intervention not amounting to war was not established beyond all doubt by the law of the League, or by the Nuremberg and Tokyo Trials. That was brought about by the law of the Charter.’⁴¹

Conclusion

Article 2(4) of the UN Charter did not merely codify an existing customary prohibition of the use of force but was rather a significant legal development which went beyond the existing laws of the time in order to found a new international legal order in the aftermath of World War II. In terms of how this position squares with the pronouncements of the majority judgment in the Nicaragua case, it must be recalled that the Court did not state that a rule of customary international law pre-existed the Charter but rather that the customary international law principle pre-existed the Charter and subsequently developed into a rule of customary international law under the Charter’s influence. Although it is not clear what legal meaning a customary international law ‘principle’ has given that this category is not recognised in article 38(1) of the Statute of the International Court of Justice, if it is understood as meaning that a legal zeitgeist was developing towards a stricter regulation of the use of force between States culminating in the prohibition set out in article 2(4) of the UN Charter, this is consistent with the historical narrative of the inter-war period outlined earlier in the chapter.

Article 2(4) as Crystallising a Rule of Customary International Law in the Process of Formation?

Another possibility for the formation of the customary prohibition of the use of force is that it was starting to emerge prior to the UN Charter and crystallised as a result of the negotiation and drafting of article 2(4). The process of crystallisation of a customary rule occurs when ‘the law evolve[s] through the practice of States on the basis of the debates and near-agreements’ revealing ‘general consensus’ during the treaty negotiation process that the rule in question is of a customary nature.⁴² This process of ‘State practice … developing in parallel with the drafting of the treaty’ is more likely to occur when the treaty negotiations and drafting take place over a long period of time,⁴³ as occurred with the new concept of the exclusive economic zone developed during the Third United Nations Conference on the Law of the Sea (1973–1982) and its acceptance by States as customary international law prior to the adoption and entry into force of the 1982 UN Convention on the Law of the Sea in 1994.⁴⁴

However, article 2(4) of the Charter arguably did not ‘crystallise’ a rule of customary international law in the process of formation, because any pre-existing customary limitations on recourse to force were significantly broadened by the advent of article 2(4), and the process of drafting was not accompanied by meaningful State practice ‘developing in parallel with’ this radical change in the law. First of all, the relevant period for crystallisation of a customary rule – the period of treaty negotiation and drafting prior to signing of the UN Charter – was extremely brief ‘due to the special circumstances occasioned by the war’.⁴⁵ ‘The constitutive instrument of the UN was conceived, negotiated, drafted, signed, and ratified in four phases, corresponding closely with events of the war … it was only towards the end of the first phase and at the beginning of the second phase [the summer of 1944] that a diplomatic exchange of ideas was set in motion.’⁴⁶ The UN Charter was then adopted on 25 June 1945 and entered into force on 24 October of the same year.

Furthermore, the term ‘use of force’ in article 2(4) was deliberately chosen by the drafters of the UN Charter to go beyond the earlier (failed) attempts to outlaw ‘war’ in the League Covenant and the Kellogg–Briand Pact, which had left open the possibility for States to claim that no war had been formally declared or officially recognised and that forcible measures fell short of war and were therefore permissible.⁴⁷ Of course, this gap between the pre-Charter prohibition of war and the prohibition of ‘use of force’ in article 2(4) is not itself an obstacle to crystallisation of any nascent customary prohibition, but it brings into stark relief that State practice (i.e. ‘the reactions of Governments to the negotiations and consultations during the work in progress’⁴⁸ or ‘repeated practice by the States concerned’⁴⁹) did not parallel this radical legal development in the treaty during the brief negotiation process.

In particular, the reaction of States to article 2(6) of the UN Charter during the drafting process clearly illustrates that they did not already accept the rule in article 2(4) as a binding rule of customary international law during the period of drafting and negotiation. Article 2(6) provides that the United Nations ‘shall ensure that states which are not Members of the United Nations act in accordance with [the Principles in article 2] so far as may be necessary for the maintenance of international peace and security’. The travaux préparatoires for this provision indicate that the delegates did not believe that they were imposing a customary obligation onto non-Members but rather that they were seeking a way to impose treaty obligations on non-treaty parties for the purpose of maintaining international peace and security as part of the new international order. The Report of the Rapporteur of the relevant Subcommittee of the San Francisco Conference stated:

The vote was taken on the understanding that the association of the United Nations, representing the major expression of the international legal community, is entitled to act in a manner which will insure the effective co-operation of non-Member states with it, so far as that is necessary for the maintenance of international peace and security.⁵⁰

Furthermore, as Kelsen highlights:

In the discussion of this paragraph at the 12th meeting of Committee I/I (U.N.C.I.O. Doc. 810, I/I/30, p.7) ‘The Delegate of Uruguay asked for a clarification of the meaning of this paragraph. He asked how a non-Member could be brought within the sphere of the Organisation and how the Organisation could impose duties upon non-Members. The Rapporteur replied that the paragraph was intended to provide a justification for extending the power of the Organisation to apply to the actions of non-Members, but that the wording might have to be reconsidered if it were not clear. … The Australian Delegate agreed that this was a difficult provision to enforce but that it was an essential one. The Organisation would have to see that everything possible would be done to suppress an aggressor.’⁵¹

During the discussions regarding article 2(6), States did not refer to a customary obligation to refrain from the use of force but, to the contrary, showed consternation about the legal basis for imposing this obligation in the UN Charter onto non-Member States. This could only be the case if States did not already accept that it was a binding rule of customary international law at the time of drafting the UN Charter. This weighs strongly against any crystallisation of a customary prohibition of the use of force in statu nascendi during the drafting and conclusion of article 2(4) of the UN Charter. Although the travaux préparatoires relating to article 2(6) are evidence that at the time of drafting and negotiation of the UN Charter, the prohibition of the use of force in article 2(4) was not accepted as a customary rule by States, it is evidence that States sought to establish a new customary rule through the impact of the UN Charter. This nuanced distinction illustrates that although crystallisation of an emerging customary rule and the development of a new customary rule triggered by a new treaty rule are ‘distinct processes, in a given case, they may shade into one another’.⁵² The significance of article 2(6) for the generation of the customary prohibition of the use of force is discussed further in Chapter 2.

Conclusion

Since article 2(4) of the UN Charter was more restrictive than pre-existing customary international law, it was not declaratory of pre-existing customary international law. For the reasons set out earlier, nor did it crystallise customary international law in the process of formation. Therefore, the customary rule prohibiting recourse to force between States must have arisen after the Charter entered into force. This is consistent with the finding of the ICJ in the Nicaragua case, as the Court did not posit that article 2(4) was declaratory of pre-existing customary international law but that the principle of the prohibition already existed under customary international law and subsequently developed under the influence of the Charter. There are two possibilities for the way this process occurred: either the new rule of customary international law developed in the usual way (State practice accompanied by an opinio juris), or article 2(4) gave rise to a new rule of customary international law through its own impact. These possibilities are discussed in Chapter 2.

2 Baxter’s Paradox and the Customary Prohibition of the Use of Force

Introduction

As we excluded in Chapter 1 the first two possibilities for the emergence of the customary prohibition of the use of force (pre-existing custom and crystallisation), let us now turn to the remaining two options for how the customary norm emerged after the advent of the UN Charter in 1945. This brings us into the realm of Baxter’s paradox and the imaginative alternative proposed by the International Court of Justice (ICJ) in the North Sea Continental Shelf Cases. This chapter accordingly explores the challenges of separately adducing the content of the parallel customary prohibition of the use of force in the presence of the parallel near-universal treaty obligation in article 2(4). Delving into these theoretical issues is not only an intriguing intellectual exercise but, as we saw in Chapter 1, fundamental to discerning the relationship between the customary and Charter prohibitions of the use of force, and in turn, the appropriate method for interpreting the meaning of prohibited force under international law (i.e. whether to focus on custom, the treaty or some combination of the two). The conclusions drawn from this chapter lay the foundation for the method that will be applied in the rest of the book to uncover the meaning of prohibited force in international law.

Challenges of the Two-Element Approach

The third option for how the customary prohibition of the use of force emerged is also the mainstream approach¹ to establishing the existence and content of a rule of customary international law, namely, the two constituent element approach: a general practice that is accepted as law.² This was the approach of the ICJ in the North Sea Continental Shelf Cases, when it held:

[T]wo conditions must be fulfilled. Not only must the acts concerned amount to a settled practice, but they must also be such, or be carried out in such a way, as to be evidence of a belief that this practice is rendered obligatory by the existence of a rule of law requiring it. The need for such a belief, i.e., the existence of a subjective element, is implicit in the very notion of the opinio juris sive necessitatis. The States concerned must therefore feel that they are conforming to what amounts to a legal obligation.³

The two-element approach has been adopted by the International Law Commission (ILC) Committee on the Identification of Customary International Law.⁴ The ILC Committee stated that each element must be separately ascertained by assessing the evidence for each element.⁵ The ILC Committee Special Rapporteur clarified that ‘the existence of one element cannot be deduced from the existence of the other’.⁶

Although this is the widely accepted approach to the identification of customary international law, it is uniquely difficult to apply to the customary prohibition of the use of force due to the presence of the parallel and near-universal treaty obligation in article 2(4) of the UN Charter. The main issue is that it is difficult to identify sufficient relevant State practice and opinio juris outside the treaty. Whether such practice and opinio juris ‘counts’ depends primarily on the extent to which conduct connected with a treaty is considered as relevant State practice or serves as evidence of an opinio juris. It also depends on the significance of verbal acts (including silence) and inaction as ‘practice’, and of UN General Assembly resolutions as evidence of opinio juris. Finally, it depends on the relative weight to be given to practice versus opinio juris. Establishing evidence of the customary rule and its content thus depends on a number of theoretical issues that remain unsettled or over which significant controversy exists. These factors taken together render it a highly fraught and complicated exercise to determine exactly when the customary prohibition of the use of force arose, as well as to identify the scope of the customary prohibition in a process distinct from the application and interpretation of article 2(4) of the UN Charter.

Non-treaty Practice

The first challenge in determining the scope of the customary prohibition of the use of force is that there is insufficient relevant State practice outside the UN Charter. Although usually ‘the conduct of parties to a treaty in relation to non-parties is not practice under the treaty, and therefore counts towards the formation of customary law’,⁷ article 2(4) of the UN Charter prohibits Member States of the United Nations from using force not only against each other but against any State, including non-Member States. This means that the only relevant practice outside the UN Charter is that of non-UN Member States.⁸

It is true that there is some potentially relevant practice by non-UN Member States. For instance, prior to becoming Members of the United Nations (i.e. before the UN Charter became directly binding on them), some States have declared their acceptance of the principles of the UN Charter including the prohibition of the use of force in article 2(4). In 1951, prior to becoming a Member of the United Nations in 1956, Japan ‘declar[ed] its intention … in all circumstances to conform to the principles of the Charter of the United Nations’ and ‘accept[ed] the obligations set forth in Article 2 of the Charter of the United Nations, in particular the obligations … to refrain in its international relations from the threat or use of force against the territorial integrity or political independence of any State or in any other manner inconsistent with the Purposes of the United Nations’.⁹ Prior to their membership of the United Nations, the Federal Republic of Germany and the German Democratic Republic also both agreed to settle their disputes exclusively by peaceful means and to refrain from the threat or use of force in accordance with the UN Charter.¹⁰ Similarly, Switzerland accepted the obligations in the UN Charter prior to becoming a Member of the United Nations in September 2002.¹¹ To this may be added instances of non-UN Member States refraining from the threat or use of force. The legal relevance of silence and inaction to the identification of a customary rule is discussed later in this chapter.

However, there are two problems with concluding that the conduct of non-States parties to the UN Charter (i.e. States that are not Members of the United Nations) that is consistent with the obligation in article 2(4) is evidence of the existence of the rule in customary international law. First, such conduct must still be accompanied by an opinio juris. The ICJ in the North Sea Continental Shelf Cases held that no inference could be drawn from State practice by non-parties to a convention which was consistent with a principle set out in it, since it did not in itself constitute evidence of an opinio juris.¹² But the second and main problem is that there is hardly any such relevant practice due to the nearly universal nature of the UN Charter. This renders difficult the identification of relevant practice by non-parties to the UN Charter, which in any case due to their relatively small number could hardly be described as a ‘general practice’. Since UN membership has grown over time, there have been periods in which a considerable number of States (including newly independent States) were not yet Members.¹³ But it is not their practice that is usually cited in support of the argument that the prohibition has formed a rule of customary international law due to widespread practice and opinio juris. As noted by Judge Sir Robert Jennings in his dissenting opinion in the Nicaragua case:

[T]here are obvious difficulties about extracting even a scintilla of relevant ‘practice’ on these matters from the behaviour of those few States which are not parties to the Charter; and the behaviour of all the rest, and the opinio juris which it might otherwise evidence, is surely explained by their being bound by the Charter itself.¹⁴

This was the paradox identified by RR Baxter:

[T]he proof of a consistent pattern of conduct by non-parties becomes more difficult as the number of parties to the instrument increases. The number of participants in the process of creating customary law may become so small that the evidence of their practice will be minimal or altogether lacking. Hence the paradox that as the number of parties to a treaty increases, it becomes more difficult to demonstrate what is the state of customary international law dehors the treaty.¹⁵

Clearly, the ICJ in the Nicaragua case (decided subsequent to Baxter’s famous pronouncement) ‘did not accept this reasoning, although it did not indicate how conduct relating to a treaty rule and to an identical customary law rule can be differentiated’.¹⁶ James Crawford also noted that ‘State practice requires that the Baxter paradox hold – that is, that treaty participation is not enough. Custom is more than treaty, more even than a generally accepted treaty … [yet] the coexistence of custom and treaty suggests that the Baxter paradox is not actually a genuine paradox.’¹⁷ Hugh Thirlway also argues that Baxter’s paradox is not really a paradox but

[i]t has merely a counter-intuitive element: one would expect that the more States show allegiance to a developing rule of law, by ratifying a treaty embodying it, the more easily it could be shown to have become a general customary rule. It states, or represents, in dramatic form a fact which is inconvenient for the development of international law, and its consistent application. There is no need to seek a ‘solution’ to the paradox, but rather a way of palliating that inconvenience.¹⁸

There are proposals to address this de lege ferenda,¹⁹ but de lege lata, it remains unclear how one can identify the scope of the parallel customary prohibition separately to article 2(4) of the UN Charter. It squarely raises the question of how post-treaty practice (such as treaty ratification, frequent repetition of a rule in multiple treaties and conduct by States parties to a treaty consistent with their treaty obligations) is to be taken into account in the formation of the customary rule. These issues are examined later.

Conduct Referable to the Treaty

Since there is virtually no potentially relevant State practice with respect to the prohibition of the use of force completely outside the UN Charter (essentially, only the practice of non-UN Member States, which we have seen earlier is extremely limited), the next questions are, first, whether State practice in compliance with a treaty obligation may count as relevant practice for the purpose of identifying a rule of customary international law; and second, whether and how we can determine if such practice in compliance with a treaty obligation is motivated by a belief in a legal obligation outside the treaty.

Does Conduct Consistent with Treaty Obligations Count as Practice?

The ICJ in the North Sea Continental Shelf Cases²⁰ confirmed that State practice consistent with the treaty by States parties should not be given weight for the purpose of identifying a customary rule. In that case, the ICJ discounted practice consistent with the treaty by States parties, even before the treaty entered into effect, since they were presumably ‘acting actually or potentially in the application of the Convention’. With respect to State practice consistent with treaty obligations, ‘[c]onduct which is wholly referable to the treaty itself does not count for this purpose as practice’;²¹ ‘in principle … what States do in pursuance of their treaty obligations is prima facie referable only to the treaty, and therefore does not count towards the formation of a customary rule’.²² Conduct referable to the treaty is not relevant ‘practice’ unless accompanied by an opinio juris outside the treaty, since on its own it does not provide evidence that a State is applying customary international law. It will require something additional to show that the conduct is not merely referable to the treaty but indicates that State’s belief about a customary legal obligation; this would usually require a verbal statement to show the State was not merely applying the treaty.

Are Acts in Compliance with Treaty Obligations Evidence of Opinio Juris?

Treating conduct of States parties to a treaty consistent with their treaty obligations as evidence of opinio juris for the existence of a customary rule is also problematic for the same reason explained earlier: on its own, State conduct in compliance with a treaty obligation is not evidence of a belief that the conduct is required by customary international law since the conduct is referable to the treaty.

Treaty Ratification and Repetition of a Rule in Multiple Treaties

In addition to the forms of practice described earlier, a plethora of multilateral treaties affirm the obligation to refrain from the threat or use of force, such as the UN Convention on the Law of the Sea, which provides in article 301 that ‘[i]n exercising their rights and performing their duties under this Convention, States Parties shall refrain from any threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the principles of international law embodied in the Charter of the United Nations’.²³ Do these treaty ratifications and repetition of the rule in multiple treaties count as opinio juris? The ILC ‘has found that the frequent enunciation of a provision in international treaties did not necessarily indicate that the provision had developed into a rule of customary international law’.²⁴ Similarly, draft conclusion 11, paragraph 2 of the ILC Committee on the Identification of Customary International Law provides that ‘[t]he fact that a rule is set forth in a number of treaties may, but does not necessarily, indicate that the treaty rule reflects a rule of customary international law’.²⁵ However, ‘in some cases it may be that frequent repetition in widely accepted treaties evinces a recognition by the international community as a whole that a rule is one of general, and not just particular, law. … But the test remains qualitative rather than quantitative.’²⁶ The ILC has previously relied upon treaty practice in assessing opinio juris for the purpose of identifying a rule of customary international law,²⁷ including with respect to the prohibition of the use of force, by referring to paragraphs (1) and (5) of the commentary to draft article 49 on the law of treaties (which mention the prohibition of the use of force in article 2(4) of the UN Charter).²⁸

Christian Tams notes that ‘[a]s regards the context, the Court has been unwilling to compartmentalise State conduct as belonging to one particular source of law only. Notably … it has regularly relied on the participation of States in treaties.’²⁹ Tams notes that ‘[a]ccording to Pellet, this in fact “might be the most important and frequent aspect of practice”’.³⁰ The Court in the Nicaragua case considered the actual treaty commitments to a rule prohibiting the use of force as themselves evidence of the parties expressing recognition of the validity of the rule as binding under customary international law:

In the present dispute, the Court, while exercising its jurisdiction only in respect of the application of the customary rules of non-use of force and non-intervention, cannot disregard the fact that the Parties are bound by these rules as a matter of treaty law and of customary international law. Furthermore, in the present case, apart from the treaty commitments binding the Parties to the rules in question, there are various instances of their having expressed recognition of the validity thereof as customary international law in other ways.³¹

For instance, the Court held that the US ratification of the 1933 Montevideo Convention on Rights and Duties of States, ‘Article 11 of which imposes the obligation not to recognize territorial acquisitions or special advantages which have been obtained by force’ was evidence of the US opinio juris.³² In other words, the Court viewed the ratification of a treaty containing the obligation to refrain from the use of force in international relations as evidence that the ratifying State accepted that such obligations in the treaty were already binding as a matter of customary international law.

However, to classify treaty ratification or the repetition of a treaty provision in a number of treaties as evidence of opinio juris regarding the existence of a customary rule requires further evidence that the States parties to the treaty believe that the treaty provision is also a customary rule; by ratifying a treaty, the parties to the treaty arguably intend to accept a treaty obligation.³³

Verbal Acts

Verbal Acts as Practice

Although acts connected with a treaty when carried out by States parties to that treaty do not necessarily carry weight as State practice for the purpose of identifying a rule of customary international law, verbal acts by States may in some cases constitute ‘general practice’. This is particularly relevant to our enquiry because most forms of practice with respect to the prohibition of the use of force between States in international law are verbal acts – statements, declarations, exchanges of claims and counter-claims – rather than physical acts such as the actual employment of inter-State force.³⁴ Unlike physical acts, many verbal acts explicitly refer to the customary nature of the rule. For example:

• UN General Assembly resolutions such as the 1970 Friendly Relations Declaration (discussed further below) and 1987 General Assembly Resolution 42/22. The latter resolution held that

  [e]very State has the duty to refrain in its international relations from the threat or use of force against the territorial integrity or political independence of any State, or from acting in any other manner inconsistent with the purposes of the United Nations. Such a threat or use of force constitutes a violation of international law and of the Charter of the United Nations and entails international responsibility.³⁵

  The final sentence implies that the prohibition is a rule of customary international law in addition to a treaty rule in the Charter. The resolution went on to declare that ‘[t]he principle of refraining from the threat or use of force in international relations is universal in character and is binding, regardless of each State’s political, economic, social or cultural system or relations of alliance’.³⁶ The significance of UN General Assembly resolutions as verbal acts is discussed further below.

• 1975 Helsinki Final Act (declaration on principles governing the mutual relations of States participating in the Conference on Security and Co-operation in Europe). The ICJ in the Nicaragua case described the effects of the Act as follows: ‘the participating States undertake to “refrain in their mutual relations, as well as in their international relations in general,” … from the threat or use of force. Acceptance of a text in these terms confirms the existence of an opinio juris of the participating States prohibiting the use of force in international relations.’³⁷ The Pact of Bogota (the American Treaty on Pacific Settlement) also requires the contracting parties to ‘refrain from the threat or the use of force, or from any other means of coercion for the settlement of their controversies’;³⁸

• State representations before the ICJ have asserted the customary international law nature of the prohibition, notably, for example, Nicaragua and the United States in the Nicaragua case;³⁹

• In the 1990 Charter of Paris for a New Europe, participating countries, ‘[i]n accordance with [their] obligations under the Charter of the United Nations and commitments under the Helsinki Final Act, … renew[ed] [their] pledge to refrain from the threat or use of force against the territorial integrity or political independence of any State, or from acting in any other manner inconsistent with the principles or purposes of those documents’.⁴⁰

Despite early debates about whether verbal acts count as State practice as well as physical acts,⁴¹ it is the dominant view in scholarship and jurisprudence that verbal acts do indeed count as State practice.⁴² The ILC acknowledges that ‘[p]ractice may take a wide range of forms. It includes both physical and verbal acts’⁴³ including ‘conduct in connection with resolutions adopted by an international organization or at an intergovernmental conference’.⁴⁴

The ILA Committee on Formation of Customary (General) International Law in its 2000 report also acknowledged that ‘[v]erbal acts, and not only physical acts, of States count as State practice’.⁴⁵ The ILA Committee argued that ‘[t]here is no inherent reason why verbal acts should not count as practice, whilst physical acts (such as arresting individuals or ships) should. For voluntarists, this must necessarily be so: both forms of conduct are manifestations of State will’.⁴⁶ Verbal acts recognised by the ILA Committee as forms of State practice were extensive:

Diplomatic statements (including protests), policy statements, press releases, official manuals (e.g. on military law), instructions to armed forces, comments by governments on draft treaties, legislation, decisions of national courts and executive authorities, pleadings before international tribunals, statements in international organizations and the resolutions these bodies adopt – all of which are frequently cited as examples of State practice – are all forms of speech-act.⁴⁷

Although it is recognised that verbal acts constitute a form of State practice, it is still ‘necessary to take account of the distinction between what conduct counts as State practice, and the weight to be given to it’.⁴⁸ Some argue that verbal acts carry more weight (e.g. the position explained by ILA), while others argue that physical acts carry more weight (‘talk is cheap’).⁴⁹ The weight to be given to verbal versus physical acts will depend on the circumstances of the case. Furthermore, the weight to be given to any particular conduct, whether verbal or physical, is arguably less a matter of weight in terms of the objective element of customary international law but goes towards the strength of evidence of an accompanying opinio juris. This is the underlying objection to accepting verbal acts as State practice, because verbal acts may not demonstrate the same commitment of the State to a position regarding the legality of an act under customary international law – a matter of opinio juris.

There is some debate as to whether double counting of verbal practice is permitted – that is, whether the same verbal acts may count as both State practice and evidence of opinio juris⁵⁰ – but it is widely accepted that this is permitted so long as both elements (State practice and opinio juris) are found to be present.⁵¹ This approach is advantageous, since ‘verbal acts generally provide explicit evidence of opinio juris unlike physical acts’,⁵² given that a belief underlying a physical act may need to be inferred.⁵³ ‘It cannot be assumed that the implication of a state’s physical acts is a belief that the act is lawful.’⁵⁴ Since verbal acts may be intended to promote a State’s preferred direction of legal developments (lex ferenda) rather than reflect its belief as to the actual state of the law (lex lata), caution is required when assessing verbal acts as evidence of an opinio juris.⁵⁵

Do UN General Assembly Resolutions Count as Evidence of Opinio Juris?

One form of verbal act has particular relevance for our enquiry into the customary international law status of the prohibition of the use of force and its scope: UN General Assembly resolutions. UN General Assembly resolutions and other ‘resolution[s] adopted by an international organization or at an intergovernmental conference may provide evidence for determining the existence and content of a rule of customary international law, or contribute to its development’.⁵⁶

In the Nicaragua case, the sources that the Court considered to be evidence of an opinio juris that the prohibition of the use of force is a rule of customary international law were primarily General Assembly resolutions, and in particular the 1970 Friendly Relations Declaration:

The effect of consent to the text of such resolutions cannot be understood as merely that of a ‘reiteration or elucidation’ of the treaty commitment undertaken in the Charter. On the contrary, it may be understood as an acceptance of the validity of the rule or set of rules declared by the resolution by themselves. The principle of non-use of force, for example, may thus be regarded as a principle of customary international law, not as such conditioned by provisions relating to collective security, or to the facilities or armed contingents to be provided under Article 43 of the Charter. It would therefore seem apparent that the attitude referred to expresses an opinio juris respecting such rule (or set of rules), to be thenceforth treated separately from the provisions, especially those of an institutional kind, to which it is subject on the treaty-law plane of the Charter.⁵⁷

However, Judge Roberto Ago in that case criticised the Court’s approach to identification of customary international law, stating:

There are, similarly, doubts which I feel bound to express regarding the idea which occasionally surfaces in the Judgment (paras. 191, 192, 202 and 203) that the acceptance of certain resolutions or declarations drawn up in the framework of the United Nations or the Organization of American States, as well as in another context, can be seen as proof conclusive of the existence among the States concerned of a concordant opinio juris possessing all the force of a rule of customary international law.⁵⁸

In the Nuclear Weapons Advisory Opinion, the Court noted the necessity of examining whether an opinio juris exists with respect to the normative character of the resolution:

General Assembly resolutions, even if they are not binding, may sometimes have normative value. They can, in certain circumstances, provide evidence important for establishing the existence of a rule or the emergence of an opinio juris. To establish whether this is true of a given General Assembly resolution, it is necessary to look at its content and the conditions of its adoption; it is also necessary to see whether an opinio juris exists as to its normative character. Or a series of resolutions may show the gradual evolution of the opinio juris required for the establishment of a new rule.⁵⁹

This highlights that there is no automatic equating a State voting in favour of a resolution with that State’s belief in the normative character of the resolution. States may have other (especially political) reasons for voting the way that they do. ‘Importantly, “[a]s with any declaration by a state, it is always necessary to consider what states actually mean when they vote for or against certain resolutions in international fora”. As States themselves often stress, the General Assembly is a political organ in which it is often far from clear that their acts carry juridical significance.’⁶⁰

Furthermore, it is important to take into account that unless the language of the resolution makes clear otherwise, such resolutions are usually non-binding.⁶¹ However, with the appropriate caution, UN General Assembly resolutions may indeed provide important evidence of opinio juris when the context, content and language of the resolution justify such a conclusion. Especially since the General Assembly is ‘a forum of near universal participation’,⁶² resolutions that are unanimous or passed by consensus are a particularly important source of evidence of opinio juris regarding the state of international law on a given topic, provided that they are not merely taken at face value but analysed with due care to identify whether the reasons for voting reflect a belief in the normative character of the resolution.

One particular example of a UN General Assembly resolution that serves as strong evidence of opinio juris that the content of the customary prohibition of the use of force is identical to article 2(4) of the UN Charter is Resolution 2625 (XXV), the Declaration on Principles of International Law concerning Friendly Relations and Co-operation among States in accordance with the Charter of the United Nations (‘Friendly Relations Declaration’). The UN General Assembly adopted this resolution by consensus on 24 October 1970 on the occasion of the twenty-fifth anniversary of the United Nations.

In the Friendly Relations Declaration, the UN General Assembly proclaimed:

Every State has the duty to refrain in its international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations. Such a threat or use of force constitutes a violation of international law and the Charter of the United Nations and shall never be employed as a means of settling international issues.⁶³

Principle 1 of the Declaration proclaims:

The principle that States shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State, or in any other manner inconsistent with the purposes of the United Nations.

In addition to comprising a subsequent agreement of UN Member States on the interpretation of article 2(4), the ICJ relied on the Friendly Relations Declaration in the Nicaragua case as an indication of States’ opinio juris on the existence and content of the customary prohibition of the use of force⁶⁴ due to its references to ‘all States’,⁶⁵ ‘principle’,⁶⁶ ‘every State’,⁶⁷ ‘a violation of international law and the Charter’⁶⁸ and the statement that ‘[t]he principles of the Charter which are embodied in this Declaration constitute basic principles of international law’.⁶⁹

The 1970 Friendly Relations Declaration is therefore strong evidence of opinio juris regarding the customary prohibition of the use of force and its content. However, although the Declaration and the other verbal acts set out earlier in the chapter refer to and confirm the customary nature of the prohibition of the use of force, they are less useful for identifying the precise scope of the customary rule and if it is identical to article 2(4) of the UN Charter. This is because these types of verbal acts that refer explicitly to customary international law are by their nature general and abstract rather than in response to specific incidents.

Silence and Inaction as State Practice and Evidence of Opinio Juris

A final category of potentially relevant practice for the identification of the scope and content of a customary international law rule prohibiting the use of force is silence and inaction, which presents further challenges. Much State practice that may be relevant is that of omission: refraining from the use of force in particular situations, refraining from characterising an act by another State as a use of force, and lack of protest. This section will look at the significance of silence and inaction for the identification of a rule of customary international law: is it relevant that States seem to refrain from making claims about ‘marginal’ forcible actions under the jus contra bellum framework? Is it enough that States generally refrain from using force against each other (inaction as relevant practice), coupled with an opinio juris?

This work uses the overarching category of ‘omission’ to describe both inaction and silence.⁷⁰ Within this broad category, one may distil two different types of omission. The first type is omission which may constitute State practice. The second type is omission in response to another State’s conduct, which may constitute evidence of opinio juris regarding the legality of the other State’s conduct through acquiescence. Collecting examples of inaction is senseless without an idea of what type of conduct is in fact being abstained from, and the categories of inaction are limited only by the imagination of the person identifying such examples. As such, to narrow the universe of all forms of State inaction to something meaningful for a legal analysis, the types of inaction that may be relevant to State practice fall into the following categories: inaction accompanied by explicit verbal statements that such conduct would be unlawful; abstention from types of forcible conduct whose legality is disputed; and inaction in circumstances where the expectation or possibility is raised for a particular State to act, such as where it is called on to do so or has previously asserted a right to so act, or where some States have taken that type of action but similar conduct is not adopted by other States. Collecting data relating to omission as potential evidence of opinio juris regarding the prohibition of the use of force under customary international law is more straightforward, since such silence or inaction will be in response to conduct of another State – either through a potential or actual threat or use of force, or official claims regarding the legality of certain conduct. For this category, one would need to identify forcible acts by States as well as verbal practice asserting the legality of forcible conduct and examine the response (or non-response) of third States. Under certain circumstances discussed in this section, inaction and silence may constitute State practice and evidence of opinio juris for the purposes of identifying a rule of customary international law. However, due to the nature of inaction and silence, they are often ambiguous and will require something more in order to be construed as evidence of such. In assessing whether inaction or silence in the face of forcible conduct or legal claims is evidence of an opinio juris regarding the legality of the conduct in question, one should consider whether the silent/inactive State had knowledge of the conduct, the capacity to respond, whether its interests are affected and if there is any evidence regarding the reasons for its silence or inaction.⁷¹

Omission as State Practice

Omission may count as State practice when inaction comprises abstention from conduct (such as the use of force) or silence in the form of refraining from asserting legal claims. According to Wood, this is a form of relevant State practice for the purpose of identifying a rule of customary international law, as long as it is accompanied by an opinio juris.⁷² Omission as State practice is distinguished from omission as evidence of opinio juris in that the former comprises abstention from asserting original legal claims to act in a particular manner under customary international law, whereas the latter is in response to another State’s conduct and may be interpreted as acquiescence in the legality of such.

Inaction as Practice

Inaction (in the sense of abstaining from physical action) has been variously characterised as a potential form of State practice, or as evidence of opinio juris.⁷³ For inaction to count as relevant State practice giving rise to a rule of customary international law, it must be general and accompanied by an opinio juris.⁷⁴ Examples of inaction that have been accepted as State practice include ‘refraining from exercising protection in favour of certain naturalized persons; abstaining from the threat or use of force against the territorial integrity or political independence of any State; and abstaining from instituting criminal proceedings in certain circumstances’.⁷⁵

In the North Sea Continental Shelf Cases, the ICJ cited and followed the Lotus case,⁷⁶ in which the Permanent Court of International Justice (PCIJ) held:

Even if the rarity of the judicial decisions to be found … were sufficient to prove … the circumstance alleged …, it would merely show that States had often, in practice, abstained from instituting criminal proceedings, and not that they recognized themselves as being obliged to do so; for only if such abstention were based on their being conscious of having a duty to abstain would it be possible to speak of an international custom. The alleged fact does not allow one to infer that States have been conscious of having such a duty; on the other hand, … there are other circumstances calculated to show that the contrary is true.⁷⁷

The clear problem is that in certain cases (such as the PCIJ Lotus decision), mere abstention can be too ambiguous to be treated as ‘a precedent capable of contributing to the formation of a customary rule’.⁷⁸ The ILA Committee states in its commentary that when conduct ‘is not clearly referable to an existing or potential legal rule’ (such as ambiguous omission), it should not count as a precedent unless there is additional evidence explaining that it occurred due to an opinio juris that the conduct abstained from would be unlawful under customary international law (as distinguished from other reasons for a State to abstain from conduct such as ‘lack of jurisdiction under municipal law; lack of interest; or a belief that a court of the flag State is a more convenient forum’).⁷⁹

Silence as Practice

Just as inaction may be a form of practice if accompanied by the required opinio juris, silence in certain circumstances can also be a form of State practice if it is ‘general’. The forms of silence referred to here are those that are not in response to the acts or claims of other States, since that is rather evidence of opinio juris (see later in the chapter). One example is given in Wood’s Second Report: that of the dissenting opinion of Judge Read in the Interpretation of Peace Treaties case (‘The fact that no State has adopted this position [that a State party to a dispute may prevent its arbitration by the expedient of refraining from appointing a representative on the Commission] is the strongest confirmation of the international usage or practice in matters of arbitration which is set forth above’)⁸⁰; although Wood lists this as an example of inaction as evidence of opinio juris, it seems to in fact comprise an instance of State practice through omission, rather than acquiescence in the practice of other States.

Omission as Opinio Juris

The second type of omission is inaction in response to the conduct of another State, or silence in the form of lack of verbal protest (which could include a failure to invoke a violation of article 2(4) or a failure to invoke a right to use force in self-defence in response to the original act). Such silence may be evidence of an opinio juris that the act does not fall within the scope of the prohibition of the use of force, such as acquiescence in the legal claims made by another State through that other State’s practice (including verbal practice). Wood goes so far as to note that ‘[i]naction by States may be central to the development and ascertainment of rules of customary international law, in particular when it qualifies (or is perceived) as acquiescence’.⁸¹ Drawing this conclusion with respect to particular incidents requires the same caution as mentioned earlier, since silence in itself is also ambiguous. Hence, the often stated requirement of the State failing to act, or remaining silent, in the face of an expectation that it act or in other circumstances that indicate an opinio juris.

Inaction as Opinio Juris

The ILC’s draft conclusion 10(3) on forms of evidence of acceptance as law (opinio juris) provides thus:

Failure to react over time to a practice may serve as evidence of acceptance as law (opinio juris), provided that States were in a position to react and the circumstances called for some reaction.⁸²

The accompanying statement of the Committee Chair⁸³ explains as follows:

The first condition is temporal. To be considered as expressing opinio juris, the failure to react needs to be maintained over a sufficient period of time, assessed in light of the particular circumstances. This condition is referred to by the expression ‘over time’. Second, paragraph 3 indicates that, in order for inaction to qualify as acceptance as law, the State must be in a ‘position to react’. This formulation is broad enough to cover the need for knowledge of the practice in question, but also other situations that might prevent a State from reacting, such as political pressures. Thirdly, it is also necessary that the circumstances called for some reaction. The Drafting Committee shared the view that States could not be expected to react to each instance of practice by other States. Attention is drawn to the circumstances surrounding the failure to react in order to establish that these circumstances indicate that the State choosing not to act considers such practice to be consistent with customary international law.

The main point is that inaction (failure to take action or to make verbal statements) in response to the acts of other States may be interpreted in certain circumstances as acquiescence in the practice of those other States – in other words, as giving rise to something similar to estoppel, so that the other States rely on the position apparently taken by the silent State vis-à-vis the act that it did not respond to. It is taken as given that the silent State has accepted the (implicit) assertion of legality of the acts taken by the first State, whose position may subsequently be relied on by that State as well as other States. This complies with the consent model of customary international law. Hence, the requirements that the silent State must have been aware of the conduct that it has not responded to and that there should be a reasonable expectation that it respond to that conduct, for example, that its interests are affected.

Silence as Evidence of Opinio Juris

Both inaction and silence through failure to respond to acts by other States may be a form of acquiescence. Under certain circumstances, silence in response to forcible acts by other States may be evidence of an opinio juris that those acts are not unlawful. There must either be evidence that the silence was actually motivated by an opinio juris or else the silence must have been in circumstances that give rise to an inference that the silent State acquiesces in the active State’s legal claims/actions. In the former case (of an opinio juris), the question is whether the silent State had an opinio juris that the relevant conduct was lawful. A factor that may indicate this is that the conduct affected its interests.⁸⁴ In the latter case (of acquiescence), it is relevant to ask: did the silent State act in a way calculated to or that does reasonably give rise to the perception that it was acquiescing in the relevant conduct?⁸⁵ In both cases, these factors will be relevant: first, the silent State must have knowledge of the conduct of the other State and, second, the silence must not be mainly motivated by extra-legal considerations.⁸⁶

In sum, this section has espoused the following dichotomy: ‘original’ inaction or silence as State practice (i.e. not in direct response to conduct or claims by another State) if general and accompanied by an opinio juris that such inaction/silence is either required or not prohibited by customary international law as the case may be, and silence and inaction in response to acts of other States as evidence of an opinio juris that such acts are lawful, that is, acquiescence. Ultimately, just as with other (i.e. active) conduct with respect to the prohibition of the use of force, in the absence of an explicit statement that a State is applying the customary rule, it will be hard or even impossible to discern whether the silence or inaction is referable to article 2(4) of the UN Charter. In other words, even if one determines that a particular State’s inaction (abstention from conduct including the assertion of legal claims) or silence (acquiescence in the conduct or legal claims of another State) has legal significance as practice with respect to the prohibition of the use of force, such conduct may be explained as compliance with the treaty obligation in article 2(4) of the UN Charter (and therefore relevant as subsequent practice in the application of the treaty) rather than evidence of the rule of custom or of an opinio juris. Therefore, on their own, silence and inaction, as well as active conduct that is in compliance with a State’s obligations under article 2(4) of the UN Charter, are insufficient to separately identify the existence and scope of the customary prohibition of the use of force.

Conclusion

The main evidence that establishes the existence of the customary prohibition falls into the following categories: treaty-related practice (which may include inaction) and verbal acts, including UN General Assembly resolutions. In particular, the 1970 Friendly Relations Declaration is strong evidence of opinio juris regarding the customary prohibition of the use of force and its content. To determine whether such evidence ‘counts’ towards establishing a general practice established as law raises fundamental issues, which have been highlighted earlier in the chapter. This makes it challenging to identify not only how and when the customary prohibition emerged, but the same difficulties present themselves when attempting to identify the content of the customary prohibition instead of interpreting article 2(4) of the UN Charter.

The ‘Own Impact’ of Article 2(4)

Given these challenges, the fourth way for the prohibition in article 2(4) to have given rise to a rule of customary international law – through the UN Charter’s ‘own impact’⁸⁷ – is both appealing and pragmatic. This process is an ‘exceptional case’ in which ‘it may be possible for a multilateral treaty to give rise to new customary rules (or to assist in their creation) “of its own impact” if it is widely adopted by States and it is the clear intention of the parties to create new customary law’.⁸⁸ The ICJ in the North Sea Continental Shelf Cases considered the possibility for a rule of customary international law to arise from the ‘own impact’ of a treaty, noting:

[I]t clearly involves treating that Article as a norm-creating provision which has constituted the foundation of, or has generated a rule which, while only conventional or contractual in its origin, has since passed into the general corpus of international law, and is now accepted as such by the opinio juris, so as to have become binding even for countries which have never, and do not, become parties to the Convention. There is no doubt that this process is a perfectly possible one and does from time to time occur: it constitutes indeed one of the recognized methods by which new rules of customary international law may be formed. At the same time this result is not lightly to be regarded as having been attained.⁸⁹

The ILA Committee on Formation of Customary Law in its 2000 Report offered the following justification for the Court’s pronouncement:

[T]the consent of States to a rule of customary law, whilst not a necessary condition of their being bound, is a sufficient condition. In other words, if States indicate by any means that they intend to be bound as a matter of customary law, being bound will be the consequence, so long as their intention is clear. They can evince that intention by a public statement, for instance. That being so, there is no a priori reason why they cannot instead evince it through, in conjunction with, or subsequent to, the conclusion of a treaty, provided that it is their clear intention to accept more than a merely convention norm.⁹⁰

This way of creating custom is to be distinguished from the ordinary customary process triggered by a new treaty rule, because the latter entails ‘a gradual build-up of customary law through the “traditional” process whereby the pool of States engaging or acquiescing in a practice gradually widens’,⁹¹ whereas under the ‘own impact’ process, the treaty itself generates the customary rule because States manifest their clear intention for it to do so. This also overcomes the problems discussed earlier with treating conduct connected with the treaty as relevant State practice or evidence of an opinio juris for the purposes of the two-element approach to the identification of a customary rule. The ILA Committee 2000 Report states that the prohibition of the threat or use of force in article 2(4) is a rare example of a treaty giving rise to a new customary rule of its own impact.⁹²

In the North Sea Continental Shelf Cases, the ICJ set out the following requirements for this process to occur. First, the treaty provision must be ‘of a fundamentally norm-creating character such as could be regarded as forming the basis of a general rule of law’.⁹³ The prohibition in article 2(4) can clearly be considered to meet this requirement, given that it has been recognised as the ‘cornerstone’ of the international legal order and is widely regarded as a norm of jus cogens (discussed in Chapter 3). In the North Sea Continental Shelf Cases, the ICJ found that the article in question in that case was not of a fundamentally norm-creating character for three reasons, namely, that the rule was subject to a ‘primary obligation’; that it was subject to a legally uncertain exception of ‘special circumstances’ and ‘the very considerable, still unresolved controversies as to the exact meaning and scope of this notion, must raise further doubts as to the potentially norm-creating character of the rule’; and third, the treaty permitted reservations to the article in question.⁹⁴ The problems identified by the Court in that case apply somewhat to article 2(4): it is subject to an exception of article 51 self-defence and Chapter VII enforcement measures, and there are ‘very considerable, still unresolved controversies as to the exact meaning and scope’ of the prohibition and its exceptions.⁹⁵ However, unlike that provision, it is not permitted to make reservations to article 2(4) and it is not subject to other primary obligations. Furthermore, the UN Charter itself is designed as a fundamentally important legal document aimed at universal adherence, and article 2(4) holds a central place within it. The rule in article 2(4) can therefore be considered to meet this requirement.

Second, the treaty provision must be ‘accepted as such by the opinio juris’ – that is, accepted that it is of a fundamentally norm-creating character. As set out earlier in the chapter, there is ample evidence of an opinio juris that the prohibition of the use of force set out in article 2(4) is binding on all States as a matter of customary international law. Article 2(6) of the UN Charter, which extends the obligations in article 2 to non-UN Member States, could also be viewed as evidence of an opinio juris that through article 2(4), States intended to create a new rule of customary international law binding on all States. Since the obligation in article 2(4) was not already a rule of customary international law at the time of the establishment of the UN Charter (as argued in Chapter 1), then article 2(6) appears to create a treaty obligation for non-parties.⁹⁶ Kelsen recognised this when he stated that ‘[f]rom the point of view of existing international law, the attempt of the Charter to apply to states which are not contracting parties to it must be characterised as revolutionary’.⁹⁷ Hence, ‘[i]n Article 2, paragraph 6, the Charter shows the tendency to be the law not only of the United Nations but also of the whole international community, that is to say, to be general, not only particular, international law’.⁹⁸

Of course, it is problematic to take the position that treaty parties could create obligations for non-parties without their consent,⁹⁹ but as Stefan Talmon notes:

The controversy has largely been mitigated by the fact that the principles enunciated in Art. 2(1) to (4) are today generally accepted as forming part of customary international law and some, such as the principle on the prohibition of the use of force in Art 2 (4), are even considered ius cogens and, as such, are binding on members and non-members alike.¹⁰⁰

The controversy is also avoided if it is considered that rather than directly seeking to impose a treaty obligation on non-treaty parties, the inclusion of article 2(6) in the UN Charter may indicate that the parties wished to create more than a conventional obligation through the establishment of the UN Charter. This position holds that non-UN Member States are bound by the prohibition only indirectly through the UN Charter (since they could be subject to enforcement action/sanctions for failing to comply with the relevant principles), but the source of their legal obligation is customary international law. Regardless of the significance attributed to article 2(6) of the Charter, at any rate at least by the time of the 1970 Friendly Relations Declaration (which declared the obligation in article 2(4) as applying to all States), an opinio juris was shared among States that the prohibition of the use of force was a rule applicable to all States and not only to UN Member States, that is, as a matter of customary international law.

Third, there must be a sufficient number of ratifications and accessions to imply a ‘positive acceptance of its principles’: ‘a very widespread and representative participation in the convention might suffice of itself, provided it included that of States whose interests were specially affected.’¹⁰¹ This suggests that the Court views participation in the convention through ratifications and accessions as a form of State practice for the purpose of identifying a rule of customary international law, which appears problematic, since without more, the parties by ratifying or acceding to the treaty are only accepting a conventional obligation and it does not indicate a belief that the rules expressed in the treaty are legally binding under customary international law.¹⁰² In any case, the UN Charter was signed by fifty-one founding Member States in 1945 and presently enjoys near-universal ratification, and accordingly meets this criterion.

Fourth, ‘State practice, including that of States whose interests are specially affected, should have been both extensive and virtually uniform in the sense of the provision invoked; − and should moreover have occurred in such a way as to show a general recognition that a rule of law or legal obligation is involved.’¹⁰³ This is also problematic because as explained in Chapter 1, mere compliance with a treaty obligation does not provide evidence of an opinio juris that the obligation is also one of customary international law. However, it appears that this requirement is directed at ensuring the practice is ‘sufficiently widespread and representative’.¹⁰⁴ It is difficult to apply this criterion to an obligation to refrain from conduct (i.e. the ‘use of force’), and it is unfortunately true that there have been many instances of States resorting to force against one another since 1945. However, States resorting to force in violation of article 2(4) do not usually acknowledge this but rather justify their conduct by appealing to exceptions such as the right of self-defence in article 51. As the ICJ recognised in the Nicaragua case, perfect compliance is unnecessary for a rule to be established as customary and that ‘[i]f a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then whether or not the State’s conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule’.¹⁰⁵ Furthermore, as set out in Chapter 1, the obligation to refrain from the use of force has since been reproduced in many multilateral and bilateral treaties, resolutions of the UN General Assembly and other international organisations, accepted unilaterally by States which were not at the time Members of the United Nations and is frequently recognised as a cornerstone of the international legal system.

Therefore, the fundamentally norm-creating character of the treaty obligation in article 2(4), its acceptance as such in the opinio juris (including possibly due to the effect of article 2(6)), the near-universality of the UN Charter and the extensive and virtually uniform State practice with respect to the prohibition of the use of force set out in that article may be considered to fulfil the criteria set out by the ICJ in the North Sea Continental Shelf Cases for a treaty provision to give rise to a new rule of customary international law ‘of its own impact.’

Conclusions: Are the Charter and Customary Prohibitions of the Use of Force Identical?

As the previous sections have argued, the customary prohibition of the use of force arose from article 2(4) of the UN Charter, either as a result of the normal process for the creation of a new rule of customary international law (with the challenges and caveats noted earlier) or exceptionally from the impact of the UN Charter. Due to the way the customary rule arose, it is likely to have been identical in content to the prohibition of the use of force in article 2(4) of the UN Charter at its inception and the two rules continue to exist in parallel.¹⁰⁶ States do not differentiate between the content or application of the prohibition under each source of law. Furthermore, States have not modified the customary prohibition by asserting claims that it is either narrower or broader than article 2(4). There are no statements to the effect that States differentiate between the application of the customary international law and article 2(4) treaty rules that this author is aware of. As a result, the content of the prohibition of the use of force under customary international law and article 2(4) of the UN Charter have not diverged from one another.

However, it is still possible that the scope and content of the prohibitions under each source of law could differ in some way due to the embedded nature of article 2(4) within the UN Charter and its explicit references to other provisions which may contain requirements not applicable to the customary rule. For example, article 2(4) refers to the Purposes of the United Nations. The customary prohibition could be narrower if it does not contain an obligation to refrain from the threat or use of force inconsistent with the Purposes of the United Nations, except insofar as those purposes are also principles of customary international law or general international law (i.e. logically inherent to the international legal system itself). On the other hand, the Friendly Relations Declaration and other documents mentioned earlier regarding the prohibition constituting customary international law also mention the Principles and Purposes of the UN Charter, which seems to indicate that a use of force inconsistent with those Purposes and Principles is also a violation of customary international law. In any case, it is difficult to conceive of a use of force inconsistent with such Purposes but not against the territorial integrity or political independence of another State, rendering this possible difference moot.

Another way that the prohibition of the use of force in the UN Charter could be broader than the customary prohibition would be if the procedural limitations to the self-defence exception to the prohibition set out in article 51 do not apply (or do not apply to the same degree) under customary law. For example, it is possible that at least non-UN Member States have a right of self-defence under customary international law which is not procedurally curtailed by the UN Security Council reporting requirement and the limit imposed on the right to self-defence ‘until the Security Council has taken measures necessary to maintain international peace and security’ set out in article 51, with the result that there may be greater scope to use force under customary law than under the UN Charter. But this does not affect the finding that the content of the prohibition of the use of force under custom and article 2(4) of the UN Charter are identical, because the self-defence exception to the prohibition of the use of force (either under article 51 or custom) is better understood for this purpose, not as a carve-out clause that affects the scope of the prohibition itself but a circumstance precluding wrongfulness of acts that would otherwise fall within its scope.

Even if the content is identical, the scope of application of the customary prohibition could differ from article 2(4) in respect of the subjects of the rule. It has been argued by Albrecht Randelzhofer and Oliver Dörr that unlike article 2(4) of the UN Charter, which only applies between States, under customary international law, international organisations (IOs) capable of conducting military operations are also bound by the prohibition, such as NATO, the EU, ECOWAS and the United Nations, and that many IOs already state this in their own constituting documents and ad hoc declarations, although this does not extend to individuals or groups.¹⁰⁷ This author is not aware of any State practice that has adopted the interpretation that non-State entities are directly bound by the prohibition of the use of force under customary international law and article 2(4) of the UN Charter from such IO declarations, although it is not excluded that the law could in future develop in this direction.

In conclusion, the prohibition of the use of force in article 2(4) of the UN Charter and under customary international law are likely to be presently identical in scope, although the possibility remains for future divergence. Chapter 3 will examine the consequences of this for the relationship between the treaty and customary rule, and the appropriate method for ascertaining the meaning of prohibited force under international law.

3 The Relationship between the Customary Prohibition of the Use of Force and Article 2(4) of the UN Charter

Introduction

Even if the content of the customary prohibition of the use of force and the prohibition of the use of force in article 2(4) of the UN Charter are currently identical, each source of law has a different method of interpretation and application. In order to determine which source to interpret or apply to discover the meaning of a prohibited ‘use of force’ between States under international law, this chapter will explore the current relationship between the treaty and customary prohibitions of the use of force (i.e. the effect of the parallel customary rule on the interpretation of article 2(4) and vice versa). The effect of article 2(4) on the customary international law prohibition after the emergence of the latter is more straightforward than the role of the customary rule in interpreting article 2(4). In essence, the scope of article 2(4) acts as a restraint on the contraction of the customary rule (i.e. it makes it more difficult to assert that the customary rule has changed to become more permissive/less prohibitive than the article 2(4) prohibition). This is because a State taking action that violates the prohibition of the use of force in article 2(4) and claiming that this action is not prohibited by the customary rule would still be violating its concurrent obligation under the UN Charter.

The current relationship between the Charter and customary prohibitions of the use of force is therefore best understood by looking at the way that the interpretation of the rule under article 2(4) of the UN Charter may change over time and the role that the customary rule can play in this process. In doing so, this chapter will examine the following related but distinct concepts: the use of pre-existing or subsequently developing custom to fill gaps in the treaty, the use of subsequently developing custom to informally modify the interpretation of the treaty, an evolutive interpretation of the UN Charter and informal treaty modification through subsequent practice. This chapter will argue that since the rule in article 2(4) is the origin of the customary prohibition of the use of force, it is not appropriate to use pre-existing or subsequently developing customary international law to fill gaps in interpretation of article 2(4), nor to use subsequently developing customary international law to modify article 2(4). Accordingly, due to the present relationship between the customary and Charter prohibitions of the use of force, the preferable approach to determine the meaning of prohibited force under international law is to focus on interpreting the UN Charter.

Effect of Customary Prohibition on the Interpretation of Article 2(4)

In terms of the effect of custom on treaty interpretation, customary international law rules may be used to supplement treaty interpretation by filling in gaps in the treaty.¹ The legal basis for doing so is article 31(3)(c) of the Vienna Convention on the Law of Treaties (VCLT).² This rule provides that, together with the context, ‘any relevant rules of international law applicable in the relations between the parties’ ‘shall be taken into account’ in interpreting a treaty. Such relevant rules include customary international law and treaty.³ The use of customary international law rules to supplement treaty interpretation may take the form of a static interpretation (using customary international law rules existing at the time the treaty entered into force) or dynamic interpretation (using subsequently developing customary international law rules). One may take into account subsequent legal developments when interpreting a treaty if it was the intention of the parties at the time of concluding the treaty, taking into account the text, object and purpose of the treaty and the travaux préparatoires.⁴ There is a presumption that this is the case for certain texts where they are open-ended or set out general obligations. International Court of Justice (ICJ) jurisprudence also supports this.⁵

Use of Pre-existing Customary International Law to Fill Gaps

Since a customary international law rule prohibiting force did not pre-exist the UN Charter but developed as a consequence of it and is currently identical to it, it is not sensible to fill gaps in the interpretation of article 2(4) such as the term ‘use of force’ by looking to custom. This is the key difference between the interpretation of articles 2(4) and 51 of the UN Charter and means that the reasoning behind turning to customary international law to supplement the interpretation of the provision does not apply in the same way to article 2(4) as it does to article 51. As pointed out by the ICJ in the Nicaragua case, since article 51 refers to an inherent right of self-defence, it must therefore be a pre-existing right under customary international law which arises when an ‘armed attack’ occurs. Although there is debate regarding whether article 51 of the UN Charter confers a treaty right or merely recognises the pre-existing customary right,⁶ it is not controversial that a right to self-defence pre-existed the UN Charter. Accordingly, it is appropriate to look to the content of that right under customary international law to fill gaps in the interpretation of article 51, such as the requirements of necessity and proportionality.⁷

Unlike article 51, which refers to a pre-existing customary rule (the right to self-defence), article 2(4) introduced a new rule (the prohibition of the ‘use of force’, as opposed to the prohibition of recourse to war). As the previous chapters explained, the new rule enshrined in article 2(4), though influenced by the pre-existing broader customary prohibition of recourse to war as an instrument of national policy, led to the emergence of a new customary rule. The term ‘use of force’ was not a legal term of art enshrined in customary international law prior to the UN Charter. It therefore does not make sense to look to the content of the customary prohibition of the use of force in order to interpret the treaty rule, since unlike the case of the right to use force in self-defence, the treaty provision in article 2(4) is itself the origin of the customary rule.

Use of Subsequently Developing Customary International Law to Interpret Article 2(4)

Though currently identical in scope, it is of course possible for the customary and treaty rule to diverge in the future. This ‘could result from different methods of interpretation and application appropriate for each category’.⁸ Although it is possible, it is unlikely that divergence would occur in the case of quasi-universal treaties. The main reason is that ‘[i]t is most unlikely in these cases that practice and opinio juris among the same States would distinguish conduct under the treaty from conduct in implementation of an identical rule of customary law’.⁹ Hugh Thirlway also notes that ‘the way in which customary law is formed theoretically involves awareness of, and lack of objection to, developments in the field on the part of the whole international community’.¹⁰

For our purposes, this means that developments in the customary prohibition of the use of force are at least accepted implicitly by the whole international community, most of the members of which are parties to the UN Charter, and, accordingly, the customary international law rule is unlikely to develop in a way that would directly conflict with their Charter obligations. The assertion of a new customary rule would require that States explicitly refer to a customary law justification for their acts. But there does not seem to be any evidence that States have already done this; when States make any reference to a source of the prohibition in their direct practice (claims and counterclaims attaching to actual uses of force), it is invariably also to the UN Charter. It therefore appears that the most plausible way the prohibition could change under custom only and not the UN Charter is if the prohibition is extended in a way that is clearly not covered by the terms of article 2(4) – for example, to cover uses of force by non-State entities or uses of force by a State within its own territory in a civil war, because then that conduct and opinio juris cannot be referable to the treaty provision.

Albeit unlikely, if it does occur, divergence in the scope or content of the prohibition of the use of force under article 2(4) and customary international law would lead to three possible interpretive outcomes. Firstly, it would result in separate rules from different legal sources simultaneously binding States.¹¹ Secondly, the development of a new customary rule with respect to the prohibition of the use of force could be used as an element of interpretation of article 2(4). And thirdly, the subsequent emergence of a new customary rule could be used as an element modifying the operation of article 2(4).¹² As the following discussion illustrates, these latter two possibilities – interpretation and modification – are not appropriate with respect to the prohibition of the use of force in article 2(4) of the UN Charter.

Interpreting Article 2(4) through Subsequently Evolving Custom

If the customary international law rule subsequently develops in a way that diverges from the article 2(4) rule, then it could make sense for the new customary rule to be used to interpret article 2(4) as a ‘relevant rule of international law applicable between the parties’,¹³ since it would be a rule of international law with a distinct content from article 2(4). An example of this is examining ‘the evolution of the rule through custom’.¹⁴ For instance, Olivier Corten’s approach is that ‘reliance on a novel right (A), supposedly accepted by all other States (B), would be both a customary evolution of the rule and a practice subsequently followed by the parties to the UN Charter and indicative of their agreement on the interpretation of the text’.¹⁵ James Green applies similar reasoning when he states:

It may well be that a new interpretation of the meaning of ‘force’ will evolve in the future to take into account the growing threat of cyberwarfare. Such a change would not require any alteration of Article 2(4), of course, just a reinterpretation of its terminology in customary international law, based on state practice and opinio juris in the usual way.¹⁶

However, one must be careful not to automatically conflate changes in the customary international law rule with changes in the interpretation of the treaty rule. As noted by Roberto Ago in the International Law Commission (ILC) debates on the 1966 draft Convention on the Law of Treaties, such an approach does not sufficiently distinguish between the distinct modes of interpretation and application of customary law and treaty law.¹⁷ Ago’s interventions on the ILC regarding the draft 1966 VCLT support the position that we must differentiate between these separate processes: subsequent agreement and subsequent practice as an element of treaty interpretation, and subsequently developing customary international law as an element of treaty interpretation.

Modifying Article 2(4) through Subsequently Evolving Custom

The use of subsequently evolving custom to interpret article 2(4) is problematic if it goes further than filling gaps pursuant to article 31(3)(c) of the VCLT and ostensibly modifies the interpretation of the treaty. Assuming that changes in custom would also informally modify the treaty is a controversial point that even the ILC did not venture into. The ILC ‘has alluded to the possibility that the emergence of a new rule of customary international law may modify a treaty, depending on the particular circumstances and the intentions of the parties to the treaty’.¹⁸ However, draft article 68(c) in the 1966 draft VCLT proposing that the operation of a treaty may be modified ‘[b]y the subsequent emergence of a new rule of customary law relating to matters dealt with in the treaty and binding upon all the parties’¹⁹ gave rise to objections by States, extensive discussions in the Commission, and was ultimately deleted on the recommendation of the Special Rapporteur, Sir Humphrey Waldock. The basis for the objections related to the complex relationship between custom and treaty law including priority of sources, the problem of inter-temporal law and the objection ‘to the idea that a new customary norm should necessarily over-ride a treaty provision regardless of the will of the parties’.²⁰ Ago noted that the provision conflated two issues, namely, the subsequent practice of the parties in the application of the treaty evidencing their agreement to extend or modify its operation, and a subsequently developing rule of customary international law.²¹ In essence, the Special Rapporteur observed that paragraph (c) ‘concern[s] the impact on a treaty of acts done outside and not in relation to it’.²²

In summary, in the event that the customary international law prohibition of the use of force subsequently evolved, this would not automatically change the interpretation of article 2(4) of the UN Charter. In practice, the scope of article 2(4) and the customary prohibition would diverge unless the change in the customary rule was accompanied by subsequent practice in the application of the treaty evidencing the agreement of (‘all, or nearly all’ of)²³ the parties to a new interpretation of article 2(4) in line with the new customary rule, and even then only to the extent that an informal modification of a substantive (as opposed to procedural²⁴) rule in the UN Charter is permissible. Informal modification of a treaty is generally problematic, since treaties usually contain formal requirements regarding modification or amendment.²⁵ Informal modification of the UN Charter is particularly problematic because it circumvents the formal mechanism for amendment set out in the Charter and thus potentially usurps the consent of the treaty parties.²⁶

Evolutive Interpretation of Article 2(4)

Of course, the interpretation of article 2(4) may still evolve over time through subsequent practice, within the limits posed by the text and the peremptory status of the prohibition. The terms of a treaty may be interpreted either in accordance with the circumstances prevailing at the time of its conclusion (contemporaneous interpretation) or in accordance with circumstances prevailing at the time of its application (evolutive interpretation).²⁷ Whether the interpretation of terms in a treaty changes over time depends on ‘whether or not the presumed intention of the parties upon the conclusion of the treaty was to give a term used a meaning which is capable of evolving over time’.²⁸ Indications of the parties’ intention at the time of concluding the treaty that the interpretation of terms change over time include the language used in the treaty. For example, ‘(a) Use of a term in the treaty which is “not static but evolutionary”. … (b) The description of obligations in very general terms, thus operating a kind of renvoi to the State of the law at the time of its application.’²⁹ In other words, the use of a term ‘whose meaning is inherently more context-dependent’³⁰ supports a conclusion that an evolutive interpretation was intended by the treaty parties at the time of its conclusion. The use of ‘generic’ terms or ‘the fact that the treaty is designed to be “of continuing duration”’,³¹ may also indicate an evolutive interpretation was intended.³² The subsequent agreements and practice of UN Member States under articles 31 and 32 of the VCLT also assist with determining the presumed intention of the treaty parties upon the conclusion of the treaty with respect to evolutive interpretation.³³

An evolutive interpretation of the UN Charter is justified by the drafters’ intention. The UN Charter was designed to be of continuing duration and to govern changing international circumstances. ‘The practical quality of the UN Charter as the constitution of the UN and the international community at large provides additional support for considering the Charter to be a “living instrument” which must be “capable of growth and development over time to meet new social, political and historical realities often unimagined by its framers.”’³⁴ Absent evidence to the contrary, this provides grounds to conclude that the term ‘use of force’ was intended to be subjected to evolutive interpretation in order to regulate changing circumstances and new uses of force which were not anticipated in 1945. This conclusion is supported by the approach of the ICJ in the Nicaragua case, which ‘apparently regarded the Charter provisions as dynamic rather than fixed, and thus as capable of change over time through state practice’.³⁵ As Thilo Rensmann argues: ‘The prevailing view today is that the Charter must be interpreted in a purposive-dynamic rather than an originalist-static manner.’³⁶ In particular, the term ‘use of force’ in article 2(4) of the UN Charter is very general and must be context dependent since such usages will change over time with, for example, technological developments. An evolutive interpretation of this provision is also supported by the drafter’s intention that the prohibition be all-encompassing. Accordingly, when interpreting the term ‘use of force’ in article 2(4) of the UN Charter, this work will also examine how the term is currently applied, taking into consideration the current context, not only the original interpretation at the conclusion of the UN Charter in 1945.

Evolutive Interpretation versus Treaty Modification

However, one must be careful to distinguish between the following two concepts. The first is an evolutive interpretation of the terms of a treaty justified by the drafter’s intention that its interpretation may change over time, which would allow consideration of, inter alia, subsequent agreements and practice that interpret the terms in a way different to the original interpretation at the time of conclusion of the treaty but still within the scope of potential natural meanings attaching to particular terms. A second and markedly different concept is the use of subsequent practice to amend or modify the terms of a treaty beyond the scope intended by the parties to the treaty at the time of its conclusion. The difference is that an evolutive interpretation, including one arrived at through the effect of subsequent practice in the application of the treaty, is the result of the process of treaty interpretation and clarifies the meaning of the terms of the treaty within the scope intended by the parties at the time of its conclusion. In contrast, an amendment or modification of the terms of a treaty by subsequent practice – outside the VCLT rules on treaty amendment and modification – alters the treaty terms beyond any potential scope for discretion afforded to the parties by the treaty.

The possibility of treaty modification through subsequent practice was not recognised by States at the Vienna Conference and may be considered to have been rejected with the deletion of draft article 38, which had included this possibility. The ILC Committee on Subsequent Agreement and Subsequent Practice in Relation to the Interpretation of Treaties has stated that ‘[t]he possibility of amending or modifying a treaty by subsequent practice of the parties has not been generally recognized’.³⁷ In practice, the line between evolutive interpretation and modification may, however, be a fine distinction,³⁸ and the ICJ has not set out criteria for making such a distinction.³⁹ Nolte concludes that ‘[t]he most reasonable approach seems to be that the line between interpretation and modification cannot be determined by abstract criteria but must rather be derived, in the first place, from the treaty itself, the character of the specific treaty provision at hand, and the legal context within which the treaty operates, and the specific circumstances of the case’.⁴⁰

In addition to the limits on treaty modification via subsequent agreement or practice (which remains highly controversial), there are further limitations on the modification of article 2(4) of the UN Charter through subsequent agreement or practice. These arise from the formal amendment procedure set out in the UN Charter itself, and the potential jus cogens nature of the norm. The formal amendment procedure for the UN Charter has a very high procedural threshold that is set out in articles 108 and 109(2) and is rarely used.⁴¹ These rules for formal modification supersede rules of formal treaty amendment or inter se modification set out in articles 40 and 41 of the VCLT.⁴² It is controversial whether the UN Charter may be amended by means other than the formal procedure set out in articles 108 and 109, such as through subsequent practice.⁴³ Modification of the UN Charter through a subsequent agreement outside of the procedure set out in the UN Charter is problematic due to article 103 of the Charter, which provides that ‘[i]n the event of a conflict between the obligations of the Members of the United Nations under the present Charter and their obligations under any other international agreement, their obligations under the present Charter shall prevail’.⁴⁴

However, maintaining a strict constitutional view that permits only formal amendments to the UN Charter risks delegitimising the United Nations since ‘the UN operates on the basis of a number of informally accepted rules’ differing from the original framework.⁴⁵ ‘In consequence the prevailing view assumes that under exceptional circumstances the member States possess the power to override the procedural restraints set forth in Arts 108 and 109.’⁴⁶ For example, ‘the replacement of the former Soviet Union and the Republic of China (Taiwan) by the Russian Federation and the People’s Republic of China without amendment to Art. 23 (1) of the Charter. Counting abstentions as well as affirmative votes as concurring votes under Art. 27 (3) may also be seen as an informal modification’.⁴⁷ But these examples relate to the procedural rules of the UN itself, and not to fundamental rules of the international legal order established by the UN Charter, such as the prohibition of the use of force in article 2(4).

In conclusion, using subsequent practice to interpret the UN Charter in a way that amounts to informal modification of its terms remains controversial; rather, ‘[i]t is presumed that the parties to a treaty, by an agreement subsequently arrived at or a practice in the application of the treaty, intend to interpret the treaty, not to amend or to modify it’.⁴⁸ Finally, if the prohibition of the use of force in article 2(4) is a norm of jus cogens, this sets further limits on changes to the rule through subsequent practice, subsequent treaties or the subsequent development of customary international law. This is discussed further below.

Jus Cogens and the Prohibition of the Use of Force

Jus cogens norms are peremptory norms of international law, defined in the VCLT as ‘a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character’.⁴⁹ Although the existence of jus cogens norms is now generally accepted,⁵⁰ the substantive content and source of jus cogens norms remain subject to debate.⁵¹ The distinguishing feature of jus cogens norms is their hierarchical superiority (as they override inconsistent customary international law and treaty), that they are not subject to derogation and that States cannot opt out as a persistent objector. This is sometimes justified on the basis of the moral force of the value that the norm protects.⁵² Others such as Hugh Thirlway⁵³ emphasise the non-derogable nature of the norm as a means of identifying norms of jus cogens through State practice.

Is the Prohibition of the Use of Force Jus Cogens?

The prohibition of the use of force is considered by many to be jus cogens,⁵⁴ although there is no ICJ ruling directly on this point.⁵⁵ The ILC stated in its commentary on the Draft Articles on the Law of Treaties that ‘the law of the Charter concerning the prohibition of the use of force’ is ‘a conspicuous example’ of a peremptory norm.⁵⁶ The ICJ in the Nicaragua case referred to the ILC’s statement,⁵⁷ which some argue ‘may indicate an inclination itself to move in that direction, but it does not constitute a determination to that effect’.⁵⁸ Various ICJ judges in their separate and dissenting opinions have declared that the prohibition of the use of force is a peremptory norm.⁵⁹ In his fourth report,⁶⁰ the ILC Special Rapporteur on the topic of peremptory norms of general international law (jus cogens), Dire Tladi, included the prohibition of aggression⁶¹ in an illustrative list of jus cogens norms. Tladi canvassed relevant practice in support of the ILC’s recognition of the prohibition of aggression as a peremptory norm, including the 1974 GA Definition of Aggression.⁶² The ILC Drafting Committee subsequently adopted a draft conclusion setting out an illustrative list of ‘the most widely recognised examples of peremptory norms of general international law’, which lists as the first example ‘the prohibition of aggression or aggressive force’.⁶³ Scholars arguing in favour of the prohibition of the use of force as a peremptory norm run the gamut between the position that the entire jus contra bellum is jus cogens;⁶⁴ that all of article 2(4) is jus cogens;⁶⁵ that only the prohibition of the use of force (as opposed to threats of force) in article 2(4) is jus cogens;⁶⁶ to those who take the view that only a narrow core of the prohibition (i.e. aggression) is jus cogens.⁶⁷

James Green has criticised the tendency for uncritical conclusions that the prohibition of the use of force is jus cogens and pointed out key issues with characterising the prohibition of the use of force as a peremptory norm.⁶⁸ There are two main bases for his critique. The first issue concerns the flexibility and uncertain nature of the scope and content of the jus contra bellum. Green notes that the content and scope of a peremptory norm on the use of force is very difficult to determine and that, as set out earlier in the chapter, a wide range of possibilities have been put forward by scholars.⁶⁹ This is due to the nature of the prohibition of the use of force and its scope: article 2(4) sets out two prohibitions (on the threat and use of force) and is subject to two exceptions set out in the UN Charter (article 51 and Chapter VII Security Council authorisation) as well as the ‘exception’ of valid consent. Not all of the concepts are treated in the same way in the legal discourse and practice of States – for example, the difference in treatment of threats of force and uses of force has led some scholars to argue that the prohibition of the threat of force is not even a customary norm, let alone a peremptory one.⁷⁰ In addition, each of these concepts is in turn subject to areas of uncertainty and is informed by or has its origin in different sources of international law. For example, there is continuing uncertainty over the content of the customary international law requirements of necessity and proportionality of self-defence measures,⁷¹ and contested areas of the jus contra bellum such as whether there is the right to anticipatory self-defence.⁷² This does not necessarily prevent the prohibition of the use of force from having peremptory status but requires either that the norm be framed in a broad way to include either the entire jus contra bellum⁷³ or exceptions to the prohibition of the use of force, or that the jus cogens norm be construed restrictively to confine it to the most certain areas (generally, the core of ‘aggression’).⁷⁴ Green argues that ‘the inherent uncertainty and flexibility of the prohibition would not seem to be compatible with the conception of peremptory norms as set out in the Vienna Convention on the Law of Treaties’.⁷⁵

The second issue is ‘whether there is enough evidence to establish that the prohibition of the use of force is peremptory in nature’.⁷⁶ Green argues for a positivist approach to the identification of jus cogens norms by examining State practice.⁷⁷ This accords with the ILC’s indication that ‘peremptory norms are formed as a result of a process of widespread acceptance and recognition of such norms as peremptory by the international community as a whole’.⁷⁸ Green canvasses a range of such practice that does not necessarily bear out the peremptory status of the prohibition of the use of force, observing that ‘in notable instances where states have had the opportunity to explicitly affirm the peremptory status of the prohibition, and might reasonably have been expected to do so, there has been a trend toward silence on the issue’.⁷⁹ Although most States stayed silent on this point during relevant debates in treaty negotiations, in the Sixth Committee of the General Assembly and the UN Security Council, very rarely has any State actually rejected the jus cogens status of the prohibition of the use of force, with nearly all explicit statements on this issue arguing in favour of the peremptory status of the prohibition. ‘As such, one may point to a cumulative effect of acceptance across these examples’⁸⁰ and the argument could be made that the majority of States have not explicitly affirmed the jus cogens nature of the prohibition since it is ‘self-evident’ or for political reasons.⁸¹ However, Green questions ‘whether silence is enough to bestow supernorm status on a rule’.⁸²

In conclusion, there is no consensus as to whether and to what extent the prohibition of the use of force is jus cogens. The majority position appears to be that the prohibition (or at least a small core of it) is a peremptory norm, however, this position is also subject to powerful critiques. Ultimately, as Green notes, ‘[t]he only way to reach a firm conclusion on this question is through an extensive and systematic survey of state practice’,⁸³ which is beyond the scope of the present work.

Consequences of Jus Cogens Nature of the Prohibition

If the prohibition of the use of force is jus cogens, the legal consequences for violation are more stringent. In addition to the consequences of a threat to the peace, breach of the peace or act of aggression set out in Chapter VII of the UN Charter, under customary international law, a prohibited use of force gives rise to international State responsibility and the obligation to cease the unlawful act,⁸⁴ make reparation⁸⁵ and the right of the victim State to take non-forcible countermeasures.⁸⁶ If a use of force in violation of article 2(4) is considered to be a peremptory norm, there are additional consequences of a serious breach of the prohibition, namely, that other States shall co-operate using lawful means to bring the violation to an end, shall not recognise the situation as lawful and shall not render aid or assistance in maintaining the situation.⁸⁷ If the entire prohibition of the use of force is jus cogens, then even uses of force at the lower boundary of the prohibition in terms of intensity or effects could potentially be a serious breach of a peremptory norm if it ‘involves a gross or systematic failure by the responsible State to fulfil the obligation’,⁸⁸ giving rise to these corresponding consequences.

Furthermore, if the rule in article 2(4) of the UN Charter is jus cogens, States cannot legally conclude treaties that are the result of a prohibited threat or use of force or enter into legally binding treaties that conflict with the rule. Under article 52 of the VCLT, ‘[a] treaty is void if its conclusion has been procured by the threat or use of force in violation of the principles of international law embodied in the Charter of the United Nations’. The ICJ held that this reflects customary international law in the Fisheries Jurisdiction (UK v Iceland) case.⁸⁹ Regarding the second point, article 53 of the VCLT provides that if at the time of the conclusion of a treaty, it conflicts with a jus cogens norm, then the treaty is void ab initio. One practical example of this is a treaty purporting to provide ‘prospective consent to authorize the use of force by one state against another, irrespective or against its will at the moment when force is being used’. If the prohibition of the use of force is jus cogens, then this ‘constitutes a derogation from the prohibition … Such consent embodied in a treaty or in a unilateral act would be void for its conflict with jus cogens on the basis of Article 53 VCLT and general international law.’⁹⁰ This could conceivably encompass standing authorisations under regional collective security agreements, such as article 4(h) of the Constitutive Act of the African Union,⁹¹ which recognises ‘the right of the Union to intervene in a Member State pursuant to a decision of the Assembly in respect of grave circumstances, namely: war crimes, genocide and crimes against humanity’.⁹²

The International Law Commission noted ‘a certain overlap in the application of the jus cogens provisions of … the draft articles and Article 103 of the Charter because certain provisions of the Charter, notably those of Article 2, paragraph 4, are of a jus cogens character’.⁹³ Due to the operation of article 103 of the Charter, the obligations in article 2(4) would prevail over the obligations of UN Member States under any other international agreement in the event of a conflict between the obligations. As noted by the ILC,⁹⁴ the difference is that if article 2(4) is jus cogens, then a conflicting treaty will be completely void, not merely that the obligation under the UN Charter would prevail over the conflicting obligation. In any case, if the prohibition of the use of force is in fact jus cogens, then as Thirlway notes,⁹⁵ it is unlikely that States would enter into a treaty that conflicts with this obligation and then later seek to denounce it as void on this basis.

For the purpose of identifying the meaning of a prohibited ‘use of force’ under international law, the jus cogens nature of the norm is relevant to the standard of modification, to which we will now turn.

Modification Standard of the Prohibition of the Use of Force if It Is Jus Cogens

If the prohibition of the use of force is a peremptory norm of international law, then there will be a higher standard applicable for determining whether subsequent State practice (for treaty interpretation) or State practice and opinio juris (for customary international law) has modified the scope or content of the norm. This is because a peremptory norm ‘can be modified only by a subsequent norm of general international law having the same character’.⁹⁶ Notably, the modification standard (i.e. jus cogens status of the norm) is only relevant to attempts to make the rule less restrictive, either through interpreting the rule in a way that results in a narrower scope or through new derogations or exceptions to the rule. Making the rule narrower would be inconsistent with the original (peremptory) rule, which means that the new narrow interpretation would also have to be a jus cogens rule to override the original broader interpretation.⁹⁷

Conversely, making the rule broader does not contravene the original jus cogens norm; the ‘new’ rule would preserve the original jus cogens ‘core’ of the norm and extend it under either the treaty (through an evolutive interpretation of article 2(4)) or custom (through evolving custom). In order for the part of the rule that extends beyond the original scope to also comprise jus cogens, it would have to separately meet the requirements for the development of a jus cogens norm; that is, it must also be ‘a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character’.⁹⁸ Of course, it is not necessary for an extended scope of the prohibition of the use of force to be jus cogens; it is entirely possible for only the original core to be jus cogens and for the ‘new’ part to be an ordinary treaty or customary rule. If the evolved (expanded) interpretation of the prohibition of the use of force did comprise jus cogens, then ‘any existing treaty which is in conflict with that norm becomes void and terminates’.⁹⁹

Conclusion: Which Source to Interpret or Apply?

Approaches based on analysing State practice and opinio juris in order to determine whether and how the prohibition of the use of force in article 2(4) of the UN Charter has evolved or been modified are flawed. Furthermore, since the two rules are identical in content, States do not differentiate between the two in their application of the prohibition and, most importantly, the rule in article 2(4) is itself the origin of the customary rule. It is not appropriate to use the customary prohibition to fill gaps in the interpretation of or to modify article 2(4) (unless the customary norm evolves and is used as an element of interpretation of article 2(4)). The preferable approach then to interpret the meaning of a prohibited ‘use of force’ under international law is to focus on interpreting the treaty.

There are several implications of choosing to focus on treaty interpretation to discern the meaning and content of a prohibited ‘use of force’ between States. There is no hierarchy between these different sources of law,¹⁰⁰ and even if the content of the rule under each source of law is currently identical, there are important differences in the application and interpretation of the two different sources of law:

• Relevance of State practice: The relevance of State practice differs according to the method being applied. State practice may be relevant firstly to identification of customary international law (when accompanied by an opinio juris); secondly, as subsequent practice of the parties in the application of the treaty under article 31 of the VCLT, which establishes their agreement regarding its interpretation; and, thirdly, as other subsequent practice in the application of the treaty as a supplementary means of interpretation under article 32 of the VCLT.

• Relevant State practice: Georg Nolte notes that ‘[i]t is … not always easy to distinguish subsequent agreements and subsequent practice from subsequent “other relevant rules of international law applicable in the relations between the parties” (article 31 (3) (c)). It appears that the most important distinguishing factor is whether an agreement is made “regarding the interpretation” of a treaty.’¹⁰¹ Accordingly, the main difference in method is to identify whether the State practice is in the application of article 2(4) of the UN Charter and whether such practice (in combination with other instances of State practice) establishes the agreement of the treaty parties regarding its interpretation.

• Opinio juris: Unlike the identification of the scope of the customary prohibition of the use of force, examining the interpretation of article 2(4) through subsequent practice does not require an analysis of whether acts or omissions are accompanied by an opinio juris, but only whether it is in the application of the UN Charter and if it establishes agreement of UN Member States regarding its interpretation.

• Required density of practice: The quantitative standard is probably higher for identifying whether subsequent practice in the application of the treaty evidences agreement of the parties regarding its interpretation, as this will likely require unanimity or near-unanimous agreement of all treaty parties.¹⁰²

Focusing on treaty interpretation to find the meaning of a prohibited ‘use of force’ has the advantage of avoiding the problems associated with trying to identify an evolution in the customary rule that have been noted by other scholars, such as ‘profound divergences’ over method,¹⁰³ and legal debates regarding the appropriate equilibrium ‘not only between “words” and “deeds” but also between “abstract” and “concrete” statements; between the various aspects of density of State practice (uniformity, extensiveness and duration); between the (relatively more influential) practice of powerful States and that of other members of the international community; or between the practice of the Security Council and that of the General Assembly’.¹⁰⁴ A consequence of this approach is that it does not give greater weight to the practice of more militarily powerful States. However, the practice of those more powerful States is more likely to play an influential role as a form of ‘other subsequent practice’,¹⁰⁵ since those States tend to be more active in the actual use of force and exchange of claims about the use of force, and therefore generate more relevant practice which could play a subsidiary role in interpretation (though one still needs to consider whether such practice indicates how those parties interpret the treaty). Finally, taking the UN Charter provisions as the starting point imposes certain textual constraints on the interpreter¹⁰⁶ and restricts the range of interpretive possibilities to what is offered by the text.

As Andrea Bianchi notes:

[T]here are good reasons for considering the provisions of the Charter as the starting point of the inquiry on the international legal regulation of the use of force. The first obvious reason is that there is widespread social consensus on this proposition. In most of the debates before the Security Council, in which issues of the use of force are discussed, reference is primarily made to the law of the Charter. Also in other fora the ‘official discourse’ on the use of force relies heavily on the central character of the Charter provisions.¹⁰⁷

This analysis will therefore start with the UN Charter and focus on the subsequent agreement of the parties as well as other practice in the application of the Charter as a supplementary means of interpretation, rather than seeking to identify State practice and opinio juris for the purpose of deriving the content of the rule under customary international law.