On-Side fighting in civil war: The logic of mortal alignment in Syria

On-side fighting – outright violence between armed groups aligned on the same side of a civil war’s master cleavage – represents a devastating breakdown in cooperation. Its humanitarian consequences are also grave. But it has been under-recognized empirically and therefore under-theorized by scholars to date. This article remedies the omission. Existing research can be extrapolated to produce candidate explanations, but these overlook spatial and temporal variation in on-side fighting within a war. I provide a theory that accounts for this ebb and flow. On-side fighting hinges on belligerents’ trade-offs between short-term survival and long-term political objectives. Enemy threats to survival underpin on-side cooperation; in their absence, belligerents can pursue political gains against on-side competitors. I evaluate this threat-absence theory using evidence from the ongoing Syrian Civil War’s first years. Fine-grained fatalities data capture fluctuating enemy threats to on-side groups’ survival and situate on-side fighting and its absence. Findings support threat-absence theory and contribute to research on warfighting and political competition in civil wars and to the study of coalition dynamics in other settings, including elections and legislatures.