Policymakers often trumpet the potential for third parties to stop the killing associated with civil wars, yet third parties as strategic actors also have incentives to encourage longer civil wars. We argue that in order to assess the influence of third parties on civil war duration, it is necessary to consider the interdependent nature of third party interventions as they are distributed across the set of civil war combatants. We also argue that it is important to consider the geopolitical context in which civil wars occur, rather than focusing solely on characteristics internal to these conflicts. To test our hypotheses about the impact of third parties and geopolitical factors on civil war duration, we rely on event history analysis and a sample of 152 civil wars for the period 1820–1992. We find empirical support for the idea that extremely long civil wars correspond to the equitable distribution of third party interventions—stalemates prolong wars. The analysis also indicates that separatist civil wars and ongoing civil wars in states proximate to the civil war state result in civil wars of longer duration. Finally, we find that when third parties raise the stakes of the conflict by engaging in the use of militarized force against the civil war state, the duration of these conflicts is reduced. In general, our analysis underscores the importance of modeling the interdependent and dynamic aspects of third party intervention as well as the world politics of civil wars when forecasting their duration and formulating policy.