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This essay concerns mass atrocity, not the kind that happened on September 11th, but an older kind when governments and those under them and supported by them killed innocent civilians on the basis of their ethnicity, on the basis of their politics, on the basis of their religion, or other traits of the group. These acts, crimes against humanity and genocide, were criminalized in the period after World War II by the International Military Tribunal and then by the Genocide Convention. These were very, very important steps forward in international criminal law, but the result of the post-war period was, in effect, two international criminal prescriptions, one through custom and one through treaty, and for two different crimes. The customary law crime was the crime against humanity, now codified more or less in the treaty establishing the International Criminal Court. The treaty-based law crime was the crime of genocide as set forth in the Genocide Convention of 1948.