The present chapter covers the time frame from 1918 to 1944, when the US opposed the very idea of a crime of aggression but had a leading role in the promotion of many relevant treaties concluded between the two world wars, including the Pact of Paris. Special attention will be paid to the US position during the debates within the context of the Commission on the Responsibility of the Authors of the War at the Paris Peace Conference and the United Nations War Crimes Commission.