Sizable minorities on both the left and the right want America to intervene in fewer foreign conflicts and to exercise more restraint in foreign policy. In the 2006 midterm elections, antiwar voters contributed to the Republicans’ loss of both houses of Congress. They also helped defeat Hillary Clinton in the 2008 Democratic presidential primary contest and the Republican nominee, John McCain, at that year’s general election. While McCain styled himself a “maverick,” the label could be more accurately bestowed upon the anti-interventionist Republican Ron Paul, who shocked the GOP establishment by showing that an unstinting critic of the Iraq War...