Since the end of the Cold War and the breakup of the Soviet Union we have been living in the era of American primacy. Primacy, of course, is just a euphemism for empire. The early 1990s ostensibly inaugurated a “new American Century,” with the United States bestriding the planet like a colossus. On both the left and right, policy elites fretted that absent American leadership the world would devolve into a Yeatsian “Second Coming” of anarchy, chaos, bloodshed and destruction...