This study investigates internationalism through the prism of a small European country. It explores an age in which many groups and communities - from socialists to scientists - organised themselves across national borders. Belgium was a major hub for transnational movements. By taking this small and yet significant European country as a focal point, the book critically examines major historical issues, including nationalism, colonial expansion, political activism and international relations. A main aim is to reveal the multifarious and sometimes contradictory nature of internationalism. The Belgian case shows how within one particular country, different forms of internationalism sometimes clashed and sometimes converged. The book is organised around political movements and intellectual currents that had a strong presence in Belgium. Each of the main chapters is dedicated to a key theme in European history: nationhood, empire, the relationship between church and state, political and social equality, peace, and universalism. The timeframe ranges from the fin de siècle to the interwar years. It thus covers the rise of international associations before the First World War, the impact of the conflagration of 1914, and the emergence of new actors such as the League of Nations. With its discussion of campaigns and activities that ranged beyond the nation-state, this study is instructive for anyone interested in transnational approaches to history.