With growing attention to peace-building in civil wars, scholars have increasingly focused on the role that international and regional organizations play in conflict resolution. Less attention has been paid to unilateral interventions undertaken by third-party states without the explicit consent of organizations and to the impact of unilateralism on how long civil wars last. In this article, we claim that unilateral interventions exert a cumulative impact on civil wars depending on interveners’ interrelations. States with a cooperative rapport have an easier time in bringing civil wars to an end though they act unilaterally and follow their interests in the civil war environment, whereas states that compete for influence over war combatants prolong the fighting. Analysis results from post-1945 civil wars support our expectations and show that interveners supporting opposing sides of the war increase war duration. On the other hand, third-party states bandwagoning on the same side of a civil war are effective in stopping the fighting only when the intervening parties share similar preferences.