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.

中文翻译：

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内战中的边战：叙利亚的凡人结盟逻辑

并肩战斗——在内战的主要分歧的同一边结盟的武装团体之间的彻底暴力——代表着合作的毁灭性破裂。其人道主义后果也很严重。但迄今为止，它在经验上未被充分认识，因此被学者们理论化不足。这篇文章弥补了这个遗漏。可以推断现有研究以产生候选解释，但这些研究忽略了战争中现场战斗的空间和时间变化。我提供了一个解释这种潮起潮落的理论。在边战斗取决于交战各方在短期生存和长期政治目标之间的权衡。敌人对生存的威胁是对边合作的基础；在他们缺席的情况下，交战方可以在政治上谋取利益，对抗在场竞争者。我使用正在进行的叙利亚内战最初几年的证据来评估这种没有威胁的理论。细粒度的死亡人数数据可以捕捉敌方对侧方生存威胁的波动，并确定侧方战斗及其缺席情况。研究结果支持无威胁理论，有助于研究内战中的作战和政治竞争，并有助于研究其他环境中的联盟动态，包括选举和立法机构。