Intuition suggests that major powers should be more likely to pursue joint military intervention when their preferences are most similar, but empirically, joint interventions are least likely in these cases. The solution to this puzzle lies in the strategic interaction between interveners. When states agree over policy, they face a free rider problem. A state is more willing to join an intervention as its preferences with the initial intervener diverge because doing so allows it to affect policy outcomes. To test the theory, a statistical model derived from the theoretical model is used to estimate the factors that affect the decisions of major powers to intervene in civil conflicts.