According to the Pentagon's 2008 "Base Structure Report," its annual unclassified inventory of the real estate it owns or leases around the world, the United States maintains 761 active military "sites" in foreign countries. (That's the Defense Department's preferred term, rather than "bases," although bases are what they are.) Counting domestic military bases and those on US territories, the total is 5,429.
The overseas figure fluctuates year to year. The 2008 total is down from 823 in the Pentagon's 2007 report, but the 2007 number was up from 766 in 2006. The current total is, however, substantially less than the Cold War peak of 1,014 in 1967. Still, given that there are only 192 countries in the United Nations, 761 foreign bases is a remarkable example of imperial overstretch—even more so considering that official military reports understate the actual size of the US footprint. (The official figures omit espionage bases, those located in war zones, including Iraq and Afghanistan, and miscellaneous facilities in places considered too sensitive to discuss or which the Pentagon for its own reasons chooses to exclude—e.g. in Israel, Kosovo, or Jordan.)
"The characteristic form of US power outside its territory is not colonial, or indirect rule within a colonial framework of direct control, but a system of satellite or compliant states," observes Eric Hobsbawm, the British historian of modern empires. In this sense America behaves more like the Soviet empire in Europe after World War II than the British or French empires of the 19th century.
To garrison its empire, as of last December, the United States had 510,927 service personnel (including sailors afloat) deployed in 151 foreign countries. This includes some 196,600 fighting in Iraq and 25,700 in Afghanistan.
The reach of the US military expanded rapidly after World War II and the Korean truce, when we acquired our largest overseas enclaves in the defeated countries of Germany, Italy, and Japan, and on Allied turf in Great Britain and South Korea. But despite the wartime origins of many overseas bases, they have little to do with our national security. America does not necessarily need forward-deployed military forces to engage in either offensive or defensive operations, because domestic bases are more than sufficient for those purposes. The Air Force can shuttle troops and equipment or launch bombers from continental American bases using aerial refueling, which has been standard Strategic Air Command doctrine and practice since 1951. Only after the Cold War was well under way did the Strategic Air Command expand into several overseas bases in Canada, England, Greenland, Japan, Oman, Spain, and Thailand in an effort to complicate Soviet retaliatory strategy.
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http://www.motherjones.com/politics/2008/08/americas-unwelcome-advances