The principles of separation of the jus ad bellum from the jus in bello has become an axiomata of public international law. This means that, in order to protect the application of international humanitarian law during the armed conflict, grounds of legality or illegality of the initiation of hostilities are of no concern to humanitarian law itself. Equally, from an international criminal law perspective, accountability can be brought against those who perpetrate war crimes, crimes against humanity, and genocide, but to date no justice mechanism exist to punish those who perpetrate the “supreme international crime,” the initiation of a war in violation of international law, the crime of aggression. Against the background of a quest for accountability for the illegal use of force, this Chapter will analyze both sides of the argument for and against the retention of the principle of separation vis-à-vis new legal theories of accountability for the illegal use of force, as well as some of the major doctrinal and pragmatic challenges presented to the principle of separation by certain types of military interventions, including United Nations peacekeeping and so-called “humanitarian interventions.”