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.