John Lewis Gaddis is the Robert A. Lovett professor of history and political science Yale University and Paul Kennedy is the J. Richardson Dilworth professor of history at the same school. In this NY Times Book Review interview, the two debate their views on American foreign policy. The entire piece is well worth reading, and the following are a couple of tidbits of their insights:

How Did 9/11 Change America’s Thinking About Foreign Policy?
GADDIS. The whole premise of our thinking had been that threats come from states. Then suddenly, overnight, levels of damage were done exceeding those at Pearl Harbor by a gang most of us had never heard of. That is a profound change in the national security environment. It exposes a level of vulnerability that Americans have not seen since they were living on the edge of a dangerous frontier 150 years ago.
KENNEDY. I’d agree, and then add another slant. The whole system of international law was predicated upon states. There’s no thought given in the U.N. Charter to nonstate actors. There needs to be agreement on what states can do now with threats from nonstate actors.

Does the United States Have an Empire?
GADDIS. The really important question is to look at the uses to which imperial power is put. And in this regard, it seems to me on balance American imperial power in the 20th century has been a remarkable force for good, for democracy, for prosperity. What is striking is that great opposition has not arisen to the American empire. Most empires in history have given rise to their own resistance through their imperious behavior. For most of its history as an empire, the United States did manage to be imperial without being imperious. The great concern I have with the current administration is that it has slid over into imperious behavior.
KENNEDY. John has put his finger on something very interesting, which is this dominant position of the U.S. not yet causing the emergence of counterweights. And I say ”yet” because I think there’s quite a considerable danger that it will. We now have a Europe with a larger G.D.P., and we have a China growing so fast you can hardly keep your eyes on it. Our great power status is unchallenged at the orthodox military level. But it’s beginning to look a little bit more fragmented in other dimensions.