Intervention in civil conflicts is one of the most vexing decisions facing modern great powers -- Bosnia, Kosovo, Somalia, and Haiti being merely the most recent examples. This useful book surveys the policymaking choices and the conditions that affect success or failure. Whereas in earlier decades the Cold War struggle justified intervention, the scale of social dislocation and humanitarian suffering drives policy today. By Regan's count, there have been 138 intrastate wars since the end of World War II, and outside parties have intervened in roughly two-thirds of them. The United States alone has been involved in 35. The book's statistical findings are not earthshaking: in only about 30 percent of the cases did outside action help settle the war, and intervention on behalf of the government has been more successful than that on behalf of the opposition. More interesting is Regan's finding that civil conflicts have actually diminished since the end of the Cold War. And he concludes that if states intervene multilaterally under U.N. auspices, success is most likely when the peacekeeping force is neutral and invited in by both parties.