Civil wars are the most prevalent form of large-scale violent conflict in the world today, and between one-third and one-fifth of civil wars end in rebel victories. Little systematic analysis exists, however, of how victorious rebels construct or reconstruct the state and how they govern in power, impeding efforts to resolve conflicts and prevent their recurrence. What determines the shape of the post-war state when rebels become incumbents? How do rebel movements’ relationships with civilians and the governing practices and institutions they create during civil wars affect state-society relations after rebel victory? Combining insights from studies of rebel movements, rebel governance, state-building and post-conflict governance, I explore these questions through a case study of the Frente Sandinista de Liberación Nacional in Nicaragua, tracing the evolution of the organization and its practices from insurgency through its time in power, finding that ideology and organizational structure shaped state-building plans and actions.