What is the role of religious identity in states’ decisions to intervene in foreign conflicts? Existing scholarship on external interventions in armed conflict pays little attention to religion, while many studies on religion and conflict give intrinsic importance to actors’ religious beliefs. In this article, we draw on insights from the comparative study of ethnic identity to explain foreign intervention decisions. Ethnic constructivism has been developed to explain domestic and group identity politics, but we demonstrate its utility for explaining state behavior in international politics. Based on the core premise of ethnic constructivism, we argue that coreligionism and coethnicity are poor predictors of states’ foreign policies. Rather, states create narratives of ethnic affinity in the service of political objectives. We use archival and other primary sources to test the theory's expectations through a structured within-case comparative analysis of Iran and its response to violent conflicts in Lebanon, Tajikistan, and Azerbaijan. Our findings offer robust support for our theory while providing theoretical and methodological implications for the study of “religious” and other identity-based conflicts in international politics.