Chapter 1 introduces the figure of the foreign fighter in the interwar period by focusing on the Spanish Civil War. It shows how the image of the nineteenth-century adventurer haunts the imaginary of the actors preoccupied with finding a legal status for the volunteers in Spain. This image is nonetheless constantly split in two: idealists and freebooters; heroes and opportunists; barbaric troops and brave highlanders. The chapter moves from the League to the Anglo-American doctrine, to domestic discussions and ends at The Hague in 1907. It is there that rules on foreign volunteers are codified in an international convention for the first time. The chapter further links the Brussels Conference of 1874 to those of Geneva in 1949 and offers a lens through which to understand how the shifting image of the adventurer reaches the decolonization period.