According to classic interpretations of the communist revolutions, political mobilization of peasantry was critical for the success of the revolutionary forces. This article, which reexamines the experience of civil wars in Russia, Finland, Spain, and China, argues that peasants’ contribution to the revolutions in Russia and later in China became possible under two historical conditions: breakdown of state authorities during the mass mobilization wars and existence of an unresolved agrarian problem in the countryside. Neither of these conditions alone, as the experience of other countries has shown, was sufficient for a success of the revolutionaries. The Spanish civil war of 1936–1939, for instance, was not preceded by a major international war. Because institutions of the traditional social order had not been undermined by war, Franco was able to defeat the Popular Front government, despite the peasants’ support of the revolution. In the Finnish civil war of 1918, which broke out in the wake of World War I and the Russian Revolution, state institutions did not collapse completely and the peasantry was divided in their responses to the revolution; the rural smallholders, for example, aligned with the Mannerheim's White army, not with the urban revolutionaries.