America has become an empire. Everyone says so. This is a surprise to most Americans, since few imagined that they were building such a thing. But, as historians such as Walter Nugent and Robert Kagan have recently taught us, Americans have been at this imperialist expansionism for quite some time — really since the beginning of the republic. How else to explain that the United States has gone from a handful of agrarian colonies to a world-spanning colossus in the space of only a few centuries? As you read this, American military might is deployed across the planet. The U.S....