There is a continuous tension between the United States of America, the most successful and most populous of the settler states planted by European imperialists around the world, and America's central position within the current world empire. Because the concept of "empire" has become essentially a negative epithet or an abstract historical term, understood only in relation to the more famous examples of empire creation in history. Empire, however, isn't something that the Romans invented, or that died with the British, or that is plotted by some dastardly neo-cons in the Pentagon basement. Empire is one of the most logical and inevitable of historical processes, it exists in all time periods, because it makes sense for a great many people at any given time.
Simply put, the drive for empire is the drive for unity, standardization, ease of communication and trade, the elimination of barriers between peoples, and the reduction of petty strife. It is the same urge that causes communities to evolve police forces and common systems of exchange and dispute resolution, but writ large, bringing together as much space and population as can be held together given the period's constraints in communication, military technology and economic conditions. As soon as civilization arose among the great rivers of the temperate zone, and more and more communities began living in a similar wary, producing goods that had value to their neighbors and possessed of a written language and rudimentary legal and financial structures, the drive for agglomeration began, with more and more regions uniting under the most warlike tribe in the neighborhood, giving rise to a succession of empires the achievements of which can still be seen on the world map today, from China to Europe. Sooner or later, a pattern of unification, centralization and standardization sweeps over any even mildly homogenous zone, because the benefits of such development become manifest to a great proportion of its inhabitants.
It is essential to understand the dynamics of Empire to realize that Empire is at its heart not a coercive force, that Empires do not arise by conquest alone, they arise when the governed consent to a large extent to the new structure because of the benefits it brings them. The Roman Empire did not come into being with the conquest of the Greeks and the Carthaginians, it became an Empire when Roman citizenship and its benefits was extended to 75 million people around the Mediterranean world.
The center of the empire might shift with the passage of time and new developments, as it did in China, starting in the southeast and migrating until it came to rest in Beijing, a place far north of the original civilized imperial zone, pulled there because the northern barbarians of the steppe joined the empire and eventually became the driving military force behind it. It similarly traveled from the Middle East and Egypt northward, pulled first by the advantage to communication and travel offered by the Mediterranean basin, and then by the strength of the northern barbarians, first to Greece, Carthage and Rome, then toward the Germanic tribes, settling on the western edge and then following the great colonization effort of the Europeans and ending up in the most successful and militarily capable European settler state, the United States of America. Just as the power of the Phoenicians, never able to fully exert their power because of the divisions and limitations of their cramped homeland in the Eastern Mediterranean, came to its highest flowering and greatest height of imperial unification in Carthage on the northwestern coast of Africa, so too the power of the Europeans, drained in the bloody wars for supremacy of that continent, came to rest in America where it could grow without the constraints of old borders and rivalries. From this new secure center, the empire was able to unite the old mother countries with their former colonies in a protective cocoon, defended by the ferocious and numerous soldiers bred on the North American continent and watered by the stream of capital accumulated during centuries of imperial agglomeration.