This chapter answers the first major question of the book, identifying mass killing without the justification of responding to the same as the normative core of the crime of aggression. In so doing, it debunks the standard normative account of the crime (adopted by both defenders and critics of the criminalization of aggression), which defines it as a macro wrong against a foreign state or people. It is plainly true under current international law that whether a war is criminal depends typically, although not exclusively, on which side has violated the other’s sovereignty. But that inter-state breach is not why waging such wars is criminal. It is criminal because waging war in breach of those inter-state rules entails widespread killing and the infliction of human suffering without justification. For five reasons (detailed in the chapter), this is the only internally coherent understanding of the crime. Recognizing this redefines aggression as a crime against humanity perpetrated ordinarily through a violation of sovereignty.