Civil War Contagion and Neighboring Interventions

Extant models of civil war intervention have difficulty accounting for the intervention decisions of third-party states that share a border with an ongoing civil war. This is troubling, as contiguous third parties account for a large proportion of interventions. I demonstrate that the tendency of civil wars to spread geographically pose neighbor states with threats to their well-being that are faced by no other type of intervener in the international system. Destruction, regime stability, even state survival are threatened by the prospect of civil war infection. I argue that neighboring third parties are thus motivated to intervene in an attempt to thwart war diffusion across their own borders. Through an analysis of civil war prevalence, I generate a measure of each state's yearly likelihood of being infected by a proximate civil war's hostilities. I then use this measure to explain neighboring interventions in civil wars of the post-WWII period. The results support my theorized expectations.