Since the beginning of time, war has been accompanied by atrocity. While there were attempts to regulate such violence, for most of history the penchant toward deliberate atrocity was largely viewed as a political or military problem. During World War II, however, the Allies declared that wartime atrocity was not only morally reprehensible, but also legally actionable and this declaration represented the triumph of a new paradigm for how to think about the conduct of war, the “legalist paradigm.” This Article describes the emergence of the legalist paradigm and argues that the emergence of the legalist paradigm of war was a response to the breakdown of a long-standing civilizational consensus among European Elites