The history of the Spanish Socialist Party (PSOE) during the civil war cannot be understood in isolation. The events of 1936–9 need to be placed in the context of a pre-existing internal struggle to control the party organisation. This battle set the party left – known as the left socialists or Caballeristas – against the reformist wing. The wartime behaviour of the opposing factions – and especially that of the reformists, who came to control the PSOE's national executive – needs to be analysed as a response to political developments during the period of the Second Republic (1931–6) and most especially between 1934 and 1936.

The history of the enduring and ultimately fatal division in the PSOE is one which long predated the Second Republic. Reduced to its essence, the growing division within the Spanish socialist movement concerned the validity, both in terms of principles and practical benefits, of collaboration with bourgeois political forces. The issue surfaced in a major way in the conflicting reactions to Primo de Rivera's dictatorship in the 1920s and began to have a real impact on the socialist organisation towards the end of the decade – with the crisis of the monarchy. With the arrival of the Second Republic in 1931, and with it the first genuinely representative elections, the polemic emerged from the realms of political principle to become a matter of pressing practical concern. The division was to become truly destructive as a result of the Socialists' experience of government between 1931 and 1933.