Book Chapter or Section
Military intervention into interstate and civil wars is both common and important. It lengthens wars, makes them more severe, and shapes how they are fought. Even the mere possibility of intervention can alter the course of a war as belligerent powers alter their strategies to either encourage or dissuade potential interveners. These effects of military intervention are found in both civil and interstate wars. Yet, is state intervention into interstate and civil wars essentially one phenomenon or are they distinct phenomena? By looking at which states are likely to intervene, why and when they intervene, and which wars are most likely to experience intervention, it becomes clear the similarities between state military intervention into civil and interstate wars are more significant than are the differences. In other words, despite some important differences they are subsets of the same phenomenon. In both types of wars, allies, geographically proximate states, and great powers are more likely to intervene. Also, information revealed by events within both types of wars prompts intervention and explains its timing. Last, wars in which international organizations become involved, both civil and interstate, are more likely to experience intervention. There are, however, important differences notably in the areas of cross-border ethnic ties, the presence of great powers in the war, the use of non-state proxies, and wars caused by commitment problems.
Shirkey, Zachary C. 2017. "Military Intervention in Interstate and Civil Wars: A Unified Interpretation," Oxford Encyclopedia of Empirical International Relations Theory. William R. Thompson (ed.). (Oxford: Oxford University Press).