The Great War created new challenges for the proponents of pre-1914 cosmopolitanism. This article explores this theme by studying the Inter-Parliamentary Union (IPU), an international association of members of parliament active in the interwar period. The IPU is first taken as a case study to discuss the difficulty of clearly differentiating between national politicians and agents of international civil society during the years between the wars. The article then shows how pre-war liberal internationalists had to reorient after the First World War, and how socialists and nationalists brought new agendas to the realm of international cooperation at the non-governmental level. These new perspectives shaping the international system even led to far-reaching plans for a world parliament. However, the IPU's history also shows how domestic political polarization contributed to the failure of interwar internationalism.
