Outside intervention in civil warfare is important for humanitarian, theoretical, and practical policy reasons-since 2006, much of the debate over the war in Iraq has turned on the danger of external intervention if the United States were to withdraw. Yet, the literature on intervention has been compartmented in ways that have made it theoretically incomplete and unsuitable as a guide to policy. We therefore integrate and expand upon the theoretical and empirical work on intervention and apply the results to the policy debate over the US presence in Iraq using a Monte Carlo simulation to build upon the dyadic results of probit analysis. We find that Iraq is, in fact, a significantly intervention-prone conflict in an empirical context; the prospect of a wider, regional war in the event that violence returns in the aftermath of US withdrawal cannot safely be ignored.