Chapter 1 offers a historical introduction as well as an overview of existing research in the field. It argues that by mapping out the trajectories of former volunteer soldiers, it is possible to see the many ways in which the Spanish Civil War and the broader anti-fascist engagement of the inter-war period could constitute a transformative experience and event; an event that expanded volunteers’ political horizons and gradually opened up possibilities for border-crossing political engagement in the post-war era. Thus, it sets the stage for the case studies constituting the main part of the book, showing that the political and military influence of the volunteers in Spain did not necessarily come to an end in 1938/1939 or even in 1945. In a few yet significant cases, it stretched across the globe far into the Cold War period.