That thousands of members of the International Brigades fought heroically against fascism during the Spanish Civil War is well known. Far less known—aside from the notable exception of George Orwell—is the presence of five hundred foreign combatants in the militias of the anti-Stalinist Partido Obrero de Unificación Marxista (POUM). This book provides, for the first time, an exhaustive study of these volunteers: their socio-political background, their participation in the Civil War and revolution, their role as victims of Stalinist slander and repression, and their fate during the Second World War and beyond.