This study examines the territorial restructuring of Europe between 1917 and 1923, when a radically new and highly fragile peace order was established. In a first step, it explores the peace planning efforts of Great Britain, France, and the United States in the final phase of the first world war. It then provides an in-depth view on the practice of Allied border drawing at the Paris Peace Conference of 1919. Special attention is given to a new factor in foreign policymaking, academic experts employed by the three Allied states for the tasks of peace planning and border drawing. Two case studies are presented of disputed regions where the newly drawn borders caused ethnic violence, albeit with different results: the return of Alsace-Lorraine to France in 1918-19 and the Greek-Turkish War between 1919 and 1922. A final chapter investigates the approach of the League of Nations to territorial revisionism and minority rights, thereby assessing the chances and dangers of the Paris peace order over the course of the 1920s and 1930s. The book argues that at both the international and the local levels, the temptation of violence drove key actors to simplify the acclaimed principle of national self-determination and use ethnic definitions of national identity. Local elites, administrations, and paramilitary leaders soon used ethnic notions of identity to mobilise popular support under the guise of international legitimacy. Henceforth, national self-determination ceased to be a tool of peace-making and instead became an ideology of violent resistance.