The field of international relations has long been occupied with questions around the form and composition of the nation-state and its relation to international orders. The works contained in this section reflect the ways in which the categories of population and nation, and the movement of populations across national borders, were highly contested in international thought throughout the twentieth century. Many are demonstrative of the extent to which earlier generations of thinkers understood the relationship between individual responsibilities and the actions of states in the international arena. These questions were never just academic but grounded in the desire to directly engage with and shape some of the pressing challenges facing the international order in the first half of the twentieth century: problems of racial tensions and discrimination, immigration and perceived population over-crowding, nationalist fervor and conflict.