In this superb inquiry into the reasons states use force abroad, Finnemore looks at military intervention over the past four centuries and concludes that the objectives of powerful states have evolved considerably. What the international community deems legitimate has changed over time -- and states have tended to adjust accordingly. Traditional explanations for changes in patterns of intervention emphasize the effects of new technologies or material capabilities, focusing on the costs and benefits of intervention. Finnemore does not entirely reject these accounts, but she argues that the utility of force hinges on legitimacy. In short, states calculate their interests according to what is considered acceptable. Today, for example, they try to wrap the use of force in the authority of international bodies such as the United Nations. Finnemore is not entirely successful in identifying the causes of long-term changes in norms, but she breaks new ground in showing the link between state power and purpose.