Niall Ferguson is contrary:
"It is, I discovered, acceptable among American liberals to say that the United States is an empire--provided that you deplore the fact. It is also permitted to say, when among conservatives, that American power is potentially beneficent--provided that you do not describe it as imperial. What is not allowed is to say that the United States is an empire and that this might not be wholly bad."
That's from the Harvard historian's introduction to the paperback version of Colossus: The Rise and Fall of the American Empire. He says some similar stuff in a recent article in Foreign Policy (free registration required).
Ferguson contends that the world is usually better off when there's a dominant empire than when there isn't. (Empires are older and more durable than nation-states.) His subsidiary point is that the U.S. acquired its own continental empire without having to work very hard, and for that reason and others now lacks the required grit to do its imperial duty.
But when it comes down to specifics, his analyses of Korea and Vietnam are severely flawed, in ways that make his thesis more plausible.
It seems like one could agree with Ferguson, but still not follow him to the conclusion that it would be best if we stayed in Iraq--as we would, say, under Emperor McCain, with whom Ferguson has been talking. (More at Hit and Run.) Would Ferguson really have told the Roman general whose legions were walking into a trap in the German forests in 9 AD to stay the course?