In order to qualify as a war crime, an offence must have a nexus with an armed conflict. This contextual element serves to distinguish war crimes from both ordinary crimes and other international crimes such as crimes against humanity and genocide. The case law of the international criminal tribunals reveals that this nexus requirement is an open concept, resulting in diverging interpretations by both international and domestic criminal courts. Starting from the assumption that such strong divergences are problematic from the perspective of legal certainty, this article seeks to define the nexus requirement more precisely. Those general theories that predicate the right of intervention by the international community upon the default by a state on its primary obligation to provide a certain basic level of security, offer a sound conceptual framework to identify international crimes. However, such theories are less suitable to define war crimes as a separate category within the realm of international crimes. Instead, the author proposes to reflect upon the quintessential nature of war crimes as serious violations of the laws and customs of war. By considering war crimes as perversions of accepted and legitimate conduct in warfare, it is possible to reconstruct the content and meaning of the nexus requirement.