ALex Motyl summarizes the answer to the debate on Crooked Timber both eloquently and effectively:
The United States expanded westward in a variety of distinctly imperial ways. But by eliminating local Spanish and Indian elites, extending U.S. military, legal, and administrative organizations into the acquired territories, and populating them with nonnative settlers, American built a state and not an empire. – Revolutions, Nations, Empires, p. 138.
Motyl is, in my opinion, one of the two or three best scholars working on empires in contemporary Political Science. He is also writes with exceeding clarity.
I am always astonished – although I really shouldn’t be by now – when I pick up one of his books and realize he has already anticipated most of my own arguments. It is a travesty that his books aren’t more widely read, particularly Empire, which is shamefully only available as an overpriced hardback.