This article develops a general theory regarding the onset of ethnic war, starting with two analytic innovations: a mechanism-based approach toward social facts and an emphasis on dynamic interactions. I deploy two meta-mechanisms -the security dilemma/spiral model and intergroup-intragroup interactions -as meta-synthesizers. I then bring together the numerous factors and mechanisms scattered in the literature into a more integrative and dynamic theory of ethnic war by linking factors with immediate drivers of conflictual behavior via the two meta-mechanisms. The resulting theory not only integrates numerous factors and mechanisms identified within the existing literature, but it also reveals previously hidden or neglected factors, interactions, and mechanisms that point to fruitful directions for future inquires.Since 1945, intrastate wars have been far more frequent and more destructive than interstate wars, and more than half the intrastate wars have been ethnic based rather than related to class or ideology. Not surprisingly, ethnic civil war (hereafter ethnic war), as a specific form of civil war, has become one of the most visible fields of inquiry in the social sciences, producing an already voluminous and still growing literature. 1 The study of ethnic war, however, is not without problems (for earlier critical reviews, see Wimmer, Cederman, and Min 2009). One fact stands out: the field has become extremely fragmented as it has expanded. From quantitative studies, we have many correlations that seem to link some factors with ethnic war or peace, often without causal mechanisms ). 2 From qualitative studies (i.e., comparative case studies), we have many specific theories of ethnic war that look to particular factors and causal mechanisms. Not only are syntheses lacking, but students of ethnic war tend to pit some factors and mechanisms against others, as if their favored factors and mechanisms alone can adequately explain the