Who Joins and Who Fights? Explaining Tacit Coalition Behavior among Civil War Actors

Which armed organizations form coalitions despite the inherent difficulties of cooperation in civil wars? We introduce the concept of tacit coalitions, which pertains to strategic and informal coalition behavior between civil war actors to address this puzzle. Our theoretical model of coalition behavior takes in theater-wide conflict behavior to allow for predictions that coalitions are more likely to form. It provides novel insights into the way military synergies within potential coalitions affect the trade-off between pooling resources and worrying about the division of gains. The empirical section finds considerable support for our theoretical argument that actors are more likely to engage in tacit coalition behavior (1) if potential coalitions are power balanced, (2) if joint capability of potential coalitions is not too high, and (3) when coalitions can unlock synergies. In addition, it produces evidence for the important role of geography and ethnic ties in generating military synergies.