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.

In the 1986 Nicaragua case, the International Court of Justice famously took the view that two additional criteria exist in customary international law for the exercise of collective self-defence, alongside the criteria that it shares with individual self-defence. These purported additional criteria have been commonly repeated in scholarship since. First, it is said that the state that has been attacked must ‘declare’ that it has been so attacked. Second, it must ‘request’ aid in its defence. This chapter sets out the manner in which the Court identified these requirements and whether it considered them to be legally determinative or merely evidentiary. It then goes on to examine state practice/opinio juris, to test whether the requirements indeed can be identified as rules of customary international law. It is argued that the first of those asserted requirements and declaration, in fact, has no legal basis. In contrast, it is argued that the issuance of a request is, as the Court indicated, a binding requirement for the exercise of collective self-defence.

This chapter covers the international law governing the use of force between states – the jus ad bellum. This is in contrast to the jus in bello – the law of armed conflict, or international humanitarian law – which regulates the conduct of hostilities once under way (see Chapter 11). Since at least 1945 the use of force by states has been prohibited, except in self-defence or when authorised by the United Nations Security Council. This chapter analyses the prohibition, the two exceptions, and the controversial issue of humanitarian intervention and its close relative, the ‘responsibility to protect’.

Have International Investment Agreements (IIAs) generated or changed customary law? (1) Scholarship’s optimistic approach to the status of customary international law fails to distinguish the elements of custom-creation from their proofs. (2) IIAs are not instances of expropriation or non-expropriation, hence they cannot serve as state practice. Verbal acts are not practice of the content of that verbal act. (3) IIAs are not opinio iuris because it is unlikely that treaty parties express preferences regarding customary law by concluding a treaty. (4) If treaties are an opt-out from general international law, then IIAs are likely concluded to counter perceptions of custom and a fortiori not custom-building. Therefore, a customary international law on expropriation is unlikely to be shaped significantly by IIAs and even if they did shape it, the resulting norm would be highly unspecific.