Interwar Europe was a battleground for different forms of governance, which were severely tested by civil wars that determined the old continent's future. Two paradigmatic wars, the Russian Civil War of 1917 and the Spanish Civil War of 1936, resulted in completely different scenarios: the Bolsheviks prevailed over their fierce opponents and consolidated their revolutionary state, while the Republicans of Spain were defeated by insurgent Nationalist forces. Using a systematic comparative approach, this book argues that party cohesion, professional military support, and social mobilization are three key factors that explain why some regimes survive civil war while others do not. The book further argues that administrative unity, charismatic leadership, loyalty among the military elites, and the ability to effectively mobilize the society's resources are key in determining conflict outcomes. In detailing these dynamics, the book offers important findings into the survival strategies of revolutionary regimes from then to today.