This article explores the twin phenomena of anti-fascism and transnational war volunteering through a case study of the International Group of the Durruti Column in the Spanish Civil War. This anarchist-led unit comprised approximately 368 volunteers with a variety of political views from at least 25 different countries. The article examines the relationship between these foreign volunteers and their Spanish hosts (both anarchist and non-anarchist), through, firstly, the militarization of the militias in the winter of 1936, and, secondly, the group’s role in the May Days of 1937 and its aftermath. These episodes show the often hostile attitude of Spaniards to foreigners within Spain and challenge the characterization of the conflict as distinctively internationalist. The lives of these volunteers also highlight the continuity of anti-fascism between the interwar and wartime period, with Spain acting as an ‘anti-fascist melting pot’ where volunteers of different backgrounds and political leanings came together in a common cause. This commitment, however, was not unconditional, and was frequently challenged due to circumstances within Spain. Through studying these transnational fighters, we have a more comprehensive understanding of the complex nature of twentieth century anti-fascism.