This chapter explains the development of the mechanisms to give effect to the international rule of law. The mechanism can be a complex process in which norms and institutions take shape over a considerable period of time. The chapter describes the impact of the Charter of the United Nations, as well as the Nuremberg Trial and the Tokyo Trial. It examines Kellogg-Briand Pact as the mechanism through which the idea of a general prohibition of aggressive war surfaced. The Kellogg-Briand Pact contained no provision for lapse or renunciation, and remains in force to this day. The post-Cold War, brought new actors to the scene. Initially of greatest importance were two ad hoc tribunals: the International Criminal Tribunal for the former Yugoslavia (ICTY) and the International Criminal Tribunal for Rwanda (ICTR). These were concerned with violations of international humanitarian law, and with genocide, but not with crimes against peace.