How can we describe the United States as a post-colonial society?

The United
States in the course of its short history has gone from colony to colonizer.  One could define
acts of colonization moving into French and Spanish / Mexican controlled areas on the continent
(causing Texas to fight for freedom and eventually causing war with Mexico) or later crossing
the waters to "influence" Hawaii, The Philippines, and Cuba.  These last two are
significant, as they were colonial conquests outside of the contiguous States gained during the
1898 Spanish-American War.  Indeed, in the early 1800's, the Monroe Doctrine could be
interpreted as the States stating that they alone would colonize the Western Hemisphere through
"Manifest Destiny, although both were more or less a bluff until the latter part of the
century.  

In late ancient times, Rome, the supreme power of the known world,
kept order, established trade, made civic improvements, and culture thrived under the "Pax
Romana after colonial conquest. Today, for the forseeable future, whether we like it or not,
whether we continue to violate our Founding Principles in the process, we have become "The
Policeman of the World," with a few current notable exceptions, and have instituted a
worldwide "Pax Americana.

We could therefore conclude that these United
States are indeed a post-colonial society, since there's nothing left on Earth to colonize!  Yet
it soon could be a Chinese flag joins the Stars and Bars on the Moon, and other worldly
establishments created, and colonization resumes again.