The U.S.A. operates 860 military bases in foreign countries based on 2004 figures.
source:
http://www.defenselink.mil/pubs/20040910_2004BaseStructureReport.pdfnote: The CIA operates its own private army and air force, unaccounted in the official record keeping government publications.
The NSA and other agencies operate bases overseas as well as in space and are unaccounted in the official record keeping government publications.
I consider the U.S.A. to be an empire although one spread and maintained primarily by diplomatic and economic coercion.
Both Rome and the U.S.A. began as small republics with constitutions establishing law and order for the benefit of their elite wealthy landowning classes.
Initially only wealthy male landowners of the respective republics could elect representatives and serve as lawmakers within a system of codified civil laws enforcing private property rights for the exclusive benefit of landowners, and in both republics neither females nor the lower classes could vote, own property, or serve in the Senate.
Every enterprise must grow or perish, and so it was with the agricultural landowners, who acquired more land area, more possessions, and eventually found themselves face to face with neighboring hostile savages who threatened peace and prosperity.
Armies were raised by both republics to defeat the raids from outside their territories, resulting in land conquests and the subjugation of barbarians.
The republics sent territorial governors to rule over the conquered lands, thus ensuring the continuity of trade, increasing the prosperity of the republics, and offering "membership" in the ever growing "commonwealth" to subjugated peoples in exchange for their assimilation into the culture of the conquerors.
More often than not the vast untapped resources of the conquered territories were the 'ulterior motive' used to justify a permanent military and political occupation of conquered lands and peoples, inevitably followed by official annexation of lands and inhabitants into an ever enlarging "republic".
It should be understood that Roman Publicans and American estate owners never understood what would be the ramifications and consequences of their choices and actions in pacifying the territories which were outside their borders.
They sincerely believed that the conduct of trade and the civilized order could not continue unless military force was used to defeat "the Dacians" or "the Cherokee" and neutralize their territories.
As they defeated all who threatened their way of life, the "republic" expanded, only to realize that each military success and territorial conquest could not be maintained without another conquest, another military outpost, an increasing need for more natural resources, and another territorial governor to establish "law and order" among the newly subjugated people which soon clamored for more legal rights and more money.
Both Romans and Americans considered that territorial and political expansion was "forced upon them" if civilization and culture were to be maintained in the world.
In both societies were a handful of Senators who attempted to warn their countrymen that the republic was embarking on a "path of no return" - one which would inevitably lead to corruption, despotism, and bankruptcy.
Then as now, they were shouted down by the majority as fools oblivious to the dangers threatening the republic from without.
An "empire" is always the final stage of a country's degeneration and devolution; so large, unstable, and ungovernable, that it can only be maintained abroad through military force, and can only be maintained at home through repression and the threat of force.
As social corruption, a weakening economy, and moral decline increase at home as the direct result of "empire building", an emergency situation soon emerges.
Eventually the "representative republic" must relinquish all authority and power to a dictator who inevitably demands absolute power, more territory, a larger military, and exorbitant taxation of the population in order to pay for it.
The costs of maintaining "empire" always leads to deficit spending, followed by bankruptcy, followed by complete collapse of the political and social order within the Empire.
As the old adage states:
The Road to Hell is paved with good intentions.