Once, when the writer Ian Frazier was traveling through northern Russia, his host flipped a light switch only to realize the bulb had burned out. He changed the lightbulb, and the room was illuminated. He looked at Frazier with satisfaction and said, “Ah, America!” Frazier understood this to be an example of the phenomenon wherein certain places in the world exist to those outside them as a concept, an idea. He was speaking not of America but of “America,” the feeling you get when, perhaps, something works as it should, a problem has a simple and practical solution. What does “America” mean to the world today? The foreign-policy consensus in Washington, D.C., has broken apart, and the conclusion of America’s mission in Afghanistan represents the end of an era, a two-decade post-9/11 posture. It is...