The United States is undoubtedly more virtuous than the average hegemony through history — but will that carry the day? It's one thing to be the big muscle in your neighborhood. It's another to be the local bully. What we all have to accept is the geopolitical reality of our current "unipolar world," asserts the lead essay in the summer issue of Foreign Affairs, the pre-eminent U.S. establishment journal. The political world these days has but a north pole. And this dramatically unidimensional structure of the global magnetic field is not going to be altered any time soon. Dartmouth University...