Interstate Rivals in Intrastate Conflicts: The Role of International Hostilities in Escalating Civil Wars

The growing internationalization of civil wars embeds domestic conflicts into a dense web of interstate relations. This study examines how interstate hostilities and foreign policy misalignments of external intervening states shape the intensity of civil wars. While interstate relations are commonly used to explain intervention onset, they are largely absent in studies on conflict dynamics in internationalized civil wars. I address this research gap developing a theoretical framework in which both, foreign patrons and the local clients, continuously adapt their strategies to shifting geopolitical alignments. I argue that polarized relations escalate violence by (1) incentivizing patrons to escalate support and pressure local allies into aggressive strategies, and (2) encouraging domestic belligerents to pursue costly offensives under the expectation of sustained backing. This dynamic adaptation process links the intensity of civil wars not merely to the existence of material support, but rather to the structure of interstate relations surrounding it. Using dyad-year data of all internationalized civil wars between 1989 and 2017 as well as ideal point estimates from UN General Assembly voting and a continuous measure of interstate hostilities based on ICEWS data, I find that greater enmity between opposing interveners, as well as between governments and rebel supporters, significantly increases battle-related deaths.