This chapter challenges the dominant paradigm of a ‘use of force’ in article 2(4) of the UN Charter as a coherent concept by giving examples from the subsequent agreement and subsequent practice of States showing that none of the elements of a ‘use of force’ is strictly necessary for the definition to be met. It examines anomalous examples which lack the elements of physical means, physical effects, gravity or hostile intent, taking as a basis of analysis certain acts listed in the 1974 Definition of Aggression: military occupation (article 3(a)), blockade (article 3(c)), mere presence in violation of a Status of Forces Agreement (article 3(e)) and indirect use of force through inter-State assistance (article 3(f)) or through non-State armed groups (article 3(g)). It also examines anomalous examples of non-‘use of force’: acts which appear to meet the criteria but are not characterised as such by States in their subsequent practice. These include forcible response to aerial incursion and purported maritime law enforcement. This chapter then offers possible explanations for these anomalous examples and discusses the implications for the interpretation of a prohibited ‘use of force’.

This chapter examines the meaning of a ‘use of force’ under article 2(4) of the UN Charter, focusing on its required means. It analyses whether ‘force’ in article 2(4) is restricted to particular means, namely, if it refers to physical/armed force only, if a weapon must be employed, what is considered a ‘weapon’ and if a release of kinetic energy is required. In doing so, it discusses subsequent agreements on the meaning of a ‘use of force’ in article 2(4), including the 1970 Friendly Relations Declaration, the General Assembly’s 1974 Definition of Aggression, 1987 Resolution 42/22 and the 2005 World Summit Outcome Document. It also examines in detail the travaux préparatoires of the Friendly Relations Declaration regarding the definition of ‘force’ in article 2(4) and arguments for and against a broad interpretation. This chapter concludes that ‘use of force’ article 2(4) refers only to physical force and not to non-physical forms of coercion, that it is not necessary that a ‘weapon’ be used nor is it required that kinetic energy be released, and that physical means are not essential for an act to constitute a ‘use of force’, as what counts are its physical effects.

This chapter presents and applies an original framework – type theory – to identify a prohibited ‘use of force’ between States under article 2(4) of the UN Charter, focusing on the meaning of ‘use of force’ and the contextual element of ‘international relations’. The theory of ‘type’ is firstly set out before explaining how it applies to the prohibition of the use of force. This chapter argues that a prohibited ‘use of force’ between States is characterised by a basket of elements, not all of which must be present for an act to meet the definition. Instead, these elements – including certain effects, gravity and intention – are identified and weighed to determine whether the definition of a ‘use of force’ is met. Illustrative examples are given from State practice including targeted killing and excessive or unlawful maritime law enforcement. Finally, this chapter applies the type theory framework to the testing of anti-satellite (ASAT) weapons in outer space to show how it can be applied in novel contexts. Through these case studies, this chapter draws definitive conclusions regarding the definition of a ‘use of force’ and demonstrates how to apply this framework in practice.

This conclusion sets out a definitional framework for a prohibited ‘use of force’ under article 2(4) of the UN Charter according to the type theory developed in this monograph. It sets out the contextual elements of that provision and the elements of a ‘use of force’ that are identified and discussed in greater detail in earlier parts of the monograph. Finally, it offers some reflections on the legal nature of this framework and its potential as a tool for scholars and practitioners to assess whether forcible incidents meet the threshold of a prohibited ‘use of force’ between States under international law.

This introduction highlights the prevailing uncertainty regarding the meaning of a ‘use of force’ under article 2(4) of the UN Charter and customary international law. It sets out the key research questions that this monograph addresses regarding the meaning of ‘use of force’ under jus ad bellum, including if ‘force’ means physical/armed force only and whether kinetic means or the use of particular weapons required, if a (potential) physical effect is required and the required nature of such effects, if there is a de minimis gravity threshold, and if a coercive or hostile intent is required. It also explains why the definition of prohibited force matters and its consequences under international law, including with respect to the gap between ‘use of force’ under article 2(4) and ‘armed attack’ under article 51 as well as the rise of grey zone operations. Finally, it sets out the aims and contributions of this monograph and an outline of its structure.

This chapter examines two ways the customary norm could have emerged post-1945: the two-element approach or that article 2(4) gave rise to a new customary rule of its own impact, following the approach of the ICJ in the North Sea Continental Shelf Cases. It demonstrates the challenges of applying the two-element approach 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, which makes it difficult to identify sufficient relevant State practice and opinio juris outside the treaty. Establishing evidence of the customary rule and its content in this way depends on a number of theoretical issues that remain unsettled or over which significant controversy exists. This chapter then applies the criteria set out by the ICJ in the North Sea Continental Shelf Cases to article 2(4) and argues that article 2(4) meets this test. This chapter concludes that, in contrast to the right to self-defence in article 51 of the UN Charter which explicitly has its origins in customary international law, article 2(4) is itself the origin of the customary international law prohibition of the use of force.

This chapter examines the relationship between the prohibition of the use of force in article 2(4) of the UN Charter and customary international law: if they are identical, the role the customary rule plays in the interpretation of article 2(4) and which one to interpret or apply to determine the meaning of a prohibited ‘use of force’. In doing so it examines the following 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. It argues that since article 2(4) is the origin of the customary prohibition, 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 custom to modify article 2(4). It concludes that due to the present relationship between the customary and Charter prohibitions, the preferable approach to determine the meaning of prohibited force under international law is to focus on interpreting the UN Charter.

This chapter examines the meaning of a ‘use of force’ under article 2(4) of the UN Charter, focusing on its required effects, gravity and intention. It analyses the required type of effects, namely, whether they must be physical, the required object/target, the required level of directness between the act and its harmful effects, and if temporary or potential effects count. With respect to gravity, it argues there is no de minimis threshold for a ‘use of force’ under article 2(4) but that gravity is nonetheless relevant to the contextual requirement that the act be in ‘international relations’ and is an indicative element of a ‘use of force’. Finally, it analyses whether a particular intention is required for a ‘use of force’ under article 2(4), examining accident, mistake and coercive and hostile intent. This section considers the relationship between the prohibitions of the ‘threat’ and ‘use’ of force, the non-intervention principle and the object and purpose of article 2(4). It concludes that like gravity, a coercive or hostile intent is relevant to the contextual requirements of article 2(4) and is an indicative element of a ‘use of force’.

This chapter examines the origin of the customary prohibition of the use of force between States and its relationship to article 2(4) of the UN Charter, focusing on the pre-UN Charter era. In doing so, it critically analyses two possibilities for the norm to have emerged prior to 1945: that it developed prior to the UN Charter and article 2(4) was declaratory of that pre-existing custom, or that article 2(4) crystallised a customary rule that was already in the process of formation. It rejects these two possibilities, arguing that article 2(4) was 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. Any pre-existing customary limitations on the use of force were significantly broadened by article 2(4), and the drafting process was not accompanied by meaningful State practice developing in parallel with this radical change in the law. Therefore, the customary rule prohibiting recourse to force between States must have arisen after the Charter entered into force. The emergence and development of the customary rule from 1945 onwards are examined in the next chapter.

This chapter examines the contextual elements of article 2(4) of the UN Charter and the requirement that a prohibited ‘threat or use of force’ be in ‘international relations’. It analyses the meaning of this term and whether it requires that the object of a prohibited use of force be another State and draws conclusions about the types of acts that fall within and outside the scope of this term (and thus the scope of article 2(4)). The type of acts discussed include use of force on another State’s territory or against its extraterritorial sovereign manifestations, to reclaim disputed territory not within de facto control, those in violation of international demarcation lines, those directly arising from a political dispute between States, use of force by a State within its own territory against its own population, in the exercise of law enforcement jurisdiction against private foreign actors, against entities falling short of Statehood, those with no nexus to another State (such as against an international organisation or on terra nullius) and the use of force within a State’s own territory against small-scale incursion by another State’s armed forces.