This article focuses on the importance of military factionalism in a nonconsolidated democracy: the Spanish Republic (1931–9). It builds a new micro‐dataset for over 11,000 officers during the Spanish Civil War (1936–9) to study how professional and economic interests created divisions within the military and influenced officers’ allegiances during the war. Results confirm that distributional conflicts influenced officers’ decisions in Republican‐controlled territories: officers who gained from military reforms in the years before the civil war and those with more rapid promotions in the months predating the war were more likely to remain loyal to the government. This article also explores the behavioural determinants of officers’ propensity to rebel and finds that hierarchy mattered, as senior officers influenced subordinates’ choices of side.