Many historians have downplayed the significance of interwar internationalism. They have presented the League of Nations and the campaigns of internationally-minded groups as idealistic failures in an age that was characterised by international tension and aggressive nationalisms. This book challenges such narratives by assessing transnational projects that were launched or transformed after World War One, particularly the interaction of the League of Nations with specific groups or associations. The authors reveal the different rationales and stimuli for international cooperation in this period. With fresh research from several European countries, this book makes an original contribution to the transnational history of the interwar years.