This groundbreaking study examines the degree to which public opinion in democratic countries constrains leaders when it comes to foreign policy decisions—and finds a great degree of variability. A vivid recent illustration came in the run-up to the Iraq war, when British Prime Minister Tony Blair was able to resist strong domestic opposition and commit troops to the U.S.-led invasion while other European governments were not. Baum and Potter argue that the combination of potent partisan opposition and open media makes it harder for leaders to ignore popular dissent when it comes to decisions about the use of force. The authors analyze enormous amounts of data ranging from 1965 until 2006 and find that countries where information flowed most freely—stimulated by partisan politics and open media—were the least likely to initiate conflict. They also report that countries with more political parties have tended to generate more public opposition to wars and to contribute fewer troops to “coalitions of the willing.” The book’s most striking message is that although a “democratic constraint” does exist, it is fragile: antiwar publics can often be outmaneuvered by their leaders.