This unique transnational history explores the extraordinary lives of left-wing volunteers who fought in not just one, but multiple conflicts across the globe during the mid-twentieth century. Utilising previously unpublished archival material, Heiberg, Acciai and Bjerström follow these individual soldiers through military conflicts that were, in most cases, geographically centred on individual countries but nonetheless evinced a crucial transnational dimension. From the Spanish Civil war of 1936 to the Nicaraguan Revolution of 1979, the authors marshall these diverse case studies to create a conceptual framework through which to better understand the networks and recruitment patterns of transnational volunteering. They argue that the Spanish Civil War created a model for this transnational left-wing military volunteering and that this experience shaped the global left responses to a range of conflicts throughout the twentieth century.