Preemptive wars, unilateralism, regime change, the neoconservative approach to foreign policy: Just a few months ago, powerful government officials and influential commentators presented these ideas as not just desirable but inevitable choices for a superpower confronted by unprecedented threats. With more than 900 American soldiers dead, 10,000 coalition troops wounded, a military price tag of more than $90 billion, and the main reason for going to war dismissed as a “massive intelligence failure,” these concepts lie buried in the sands of Iraq. Some of these ideas will not be missed. The reliance on military solutions alone to confront real or presumed security threats proved to be as defective an idea as deep disdain for diplomacy. Murderous chaos in postwar Iraq exposed the limits of U.S. military force, its technical superiority notwithstanding. Meanwhile, diplomacy opened possibilities embraced by once scornful and now desperate U.S. leaders who were forced to eat their words. Hopefully they learned a lesson. More fundamental, disappointments in Iraq also dealt a blow to a worldview that, for all its references to the terrorist attacks of September 11, 2001, as an epochal event, still hearkens back to the Cold War. Consider the two primary responses to the attacks on the World Trade Center and the Pentagon: Instead of concentrating all energies and resources to fight the strange, stealthy, and stateless network that perpetrated the attacks, the United States launched military assaults against two nation-states. First, it rightly attacked Afghanistan, a country whose government had been the subject of a friendly takeover by such networks. The second was Iraq, a country with a standing army and a dictator evocative of the Cold War era. Iraq offered a target more suited to the mindset of U.S. leaders and military capabilities than the more complicated terrorist...