De facto states-polities, such as Abkhazia (Georgia) or the Donetsk People's Republic (Ukraine), that appropriate many trappings of statehood without securing the status of full states-have been a constant presence in the postwar international order. Some de facto states, such as Northern Cyprus, survive for a long period of time. Others, including Tamil Eelam in Sri Lanka, are forcefully reintegrated into their parent states. Still others, such as Aceh in Indonesia, disappear as a result of peacemaking. A few, such as Eritrea, successfully transition to full statehood. What explains these very different outcomes? I argue that four factors account for much of this variation: the extent of military assistance that separatists receive from outside actors, the governance activities conducted by separatist insurgents, the fragmentation of the rebel movement, and the influence of government veto players. My analysis relies on an original dataset that includes all breakaway enclaves from 1945 to 2011. The findings enhance our understanding of separatist institutional outcomes, rebel governance, and of the conditions that sustain nonstate territorial actors. * Author's note: I am grateful to Karen Rasler, William R. Thompson, Robert Urbatsch, three anonymous reviewers, and the ISQ editors for their comments and suggestions. Supplementary materials, including the dataset, codebook, and replication files, are available on the ISQ website.