The Rome and America comparison is very much in the air these days -- you have only to cock an ear. It's invoked by those who yearn for a worldwide Pax Americana, and also by those who wring their hands about an impending "decline and fall." In truth, ancient Rome and modern America differ in a thousand ways. But a handful of parallels hold up. One has to do with the way we view the outside world. Imperial Rome often knew little about the people beyond its frontiers. Disparaging those people, it underestimated their capabilities. America's motives in the world...