Scholars argue that third parties make rational calculations and intervene to influence interstate dispute outcomes in favor of their own objectives. Third parties affect not only conflict outcomes but also escalation and duration. Theories of third-party involvement are applied to understand the dynamics of intrastate war. An analysis of event data for three Central American conflicts (El Salvador, Guatemala, and Nicaragua) from 1984 to 2001 is used to examine transnational actors' influence on the dynamics of civil war. Findings show that transnational third parties often alter levels of cooperation among domestic adversaries, and that consistency affects the strength and direction of third-party influence.