Despite their untold human suffering, widespread destruction and loss, and far-reaching destabilization, the fires of many of the world's most violent civil wars continue to burn. In light of their devastating effects, how can we explain the intractability of costly and stalemated, yet seemingly endless, civil wars? By situating internal conflicts within the broader geopolitical environment in which they take place, Wars Without End provides an answer. It highlights the critical role of competitive intervention--opposing, simultaneous transfers of military assistance from different third-party states to both government and rebel combatants--in the dynamics, duration, and global prevalence of internal conflict. Providing a comprehensive theoretical and empirical account of this form of external meddling, it brings together battlefield bargaining dynamics, the escalatory pressures of interstate competition, and the systemic dimensions of geopolitical rivalry in civil wars to explain how protracted fighting within states is linked to enduring competition between them. In doing so, it challenges traditional conceptions of "proxy war" by deriving new propositions about the strategic logics that motivate it, offering new and productive angles on the behaviors of armed groups, the strategies of foreign interveners, and the trajectories of internal wars. Combining statistical analyses with detailed case studies drawing on fieldwork, original interviews, declassified intelligence reports, and archival research, the book explains competitive intervention's pernicious effects, documents its consequences for civil wars, and proposes policy prescriptions aimed at resolving some of today's most intractable conflicts.