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 studies the methodology used by the CJEU to identify CIL. In addition, it assesses whether that methodology is similar to, or differs from, that which other international courts and tribunals use (based on the two-element approach). The chapter argues that the notion of consensus can assist the CJEU in the identification process of CIL. In particular, two sets of cases are examined. First, those in which the CJEU has dealt with the interaction between the Resolutions of the UNGA and CIL in the identification process. And secondly, cases where general principles of public international law have been used to identify CIL. Resorting to public international law concepts (consensus) and techniques (external references to the PCIJ or the ILC) to identify CIL does not prevent the CJEU from adopting a particularist approach. The judicial practice analysed highlights the tension converging when the CJEU exercises its jurisdiction. A push towards autonomy (or particularism) can be noted on the one hand. On the other hand, the CJEU cannot avoid its nature as an international tribunal. The chapter argues that it is precisely for this reason that the CJEU resorts to a ‘flexible consensualist approach’ when faced with the identification of CIL thus allowing some room for cross-fertilization between international courts and tribunals.