At a recent dinner in Cambridge, Massachusetts, four of the world’s leading historians of empire agreed on only one thing: that none of them knew what an empire is. It is a loose-fitting name, applied to states or political agglomerations of such diverse kinds that we should probably admit that entities we call empires have no more in common than girls we call Jane or garments we call coats.The best that these experts could come up with was that empires tend to be larger than other contemporary states, they consist of more than one historically distinct community, there is one central focus of allegiance, and scholars have "more or less agreed" to designate the United States as an "empire." That's it and that's all, which leads me to wonder if the concept is all that useful. Fernandez-Armesto claims that it is, but the question must be asked, especially since use of the term allows critics of the US to mindlessly plug into all sorts of Marxist and anti-colonialist analytic categories and rhetoric, whether or not such associations are appropriate.
Read his review here.