We examine how external intervention interacts with ethnic polarization to induce rebellion and civil war. Previous literature views polarization as internally produced—the result of demographic characteristics or intergroup differences made salient by ethnic entrepreneurs. We complement these approaches by showing that polarization is also affected by international politics. We model intervention’s effect on civil war via the pathway of ethnic identification—a mechanism not previously identified in the literature. In our model, local actors representing different groups are emboldened by foreign patrons to pursue their objectives violently. This, in turn, makes ethnic identity salient and induces polarization. Without the specter of intervention, polarization is often insufficient to induce war and, in turn, in the absence of polarization, intervention is insufficient to induce war. We illustrate the model with case evidence from Ukraine.