This book recovers the lost history of Spanish socialism during the turbulent years of the Civil War (1936–39). Just as the energy of the socialist movement had sustained the pre-war Second Republic as an experiment in reform, so too it underwrote the Republican war effort in the crucial years of the conflict which would determine Spain’s long-term future. Leading Socialist Party (PSOE) cadres formed the bedrock of the government, while thousands of Party and union militants helped bear the tremendous weight of the war effort. The role of the PSOE in the construction of Republican political unity during the Civil War was pivotal. Yet, paradoxically, previous accounts of wartime Republican politics have virtually written the PSOE out of the script by concentrating exclusively on the fierce ideological dispute between anarchists and communists. But the key issues of revolution and State power marked all the forces in Republican Spain, none more so than the Socialist movement. As the traditional party of the working class and the only mass party in Spain as late as 1931, PSOE militants were to be found on both sides of the revolutionary/reformist divide which split fatally the Republican forces during the Civil War. The PSOE’s disintegration was a function of that of the Republic itself; but the reverse was no less true. The book investigates the responses of organised socialism to the complex issues raised by the conflict, as it charts the PSOE’s devastating experience of political power and desperate crisis in a war it could not win.