Genocide is widely seen as a phenomenon of domestic politics, which becomes of international significance because it offends against international law. Hence there are as yet inadequate International Relations analyses of the production of genocide. This article challenges the idea of the domestic genesis of genocide, and critiques the corresponding approach of 'comparative genocide studies' which is dominant in the field. It analyses the emergence of more fruitful 'relational' and 'international' approaches in critical genocide studies, while identifying the limitations of their accounts of the 'international system'. As first steps towards an adequate international account, the article then explores questions of the international meaning and construction of genocidal relations, and of international relations as the context of genocide. It argues for a historical and sociological approach to the international relations of genocide, and examines 20th-century European genocide in this light. Arguing for a broader conception of this historical experience than is suggested by an exclusive focus on the Holocaust, the article offers an interpretation of genocide as increasingly endemic and systemic in international relations in the first half of the century. It concludes by arguing that this account offers a starting point, but not a model, for analyses of genocide in global international relations in the 21st century.