The U.S. is expanding its military presence in Colombia, which prompts a look at our ever-growing overseas troop presence and what it is costing us—not just in money.

The Institute for Policy Studies finds that we have at least 865 bases outside the United States, and that more than a third of American troops "are currently based abroad or afloat in international waters." Because some bases are secret and because the term "bases" applies ambiguously to some installations, it's hard to arrive at a definitive number; for example, some facilities in Saudi Arabia are not officially listed as bases, though that is their real use.

The cost of maintaining the bases, including meeting the needs of the troops (and their dependents and others), comes to some $250 billion a year ($272 billion in 2009), the IPS estimates. It's a bill we can ill afford, says the group's report, and at least a part of the money would be better spent cooperating with other countries to solve common security problems.

Chalmers Johnson, a Far East historian and past consultant to the CIA, argues in his book The Sorrows of Empire that what he calls the "empire of bases" needs to be cut back, difficult as that may be. He quotes a statement from the Senate Foreign Relations Committee in 1970: "Once an American overseas base is established it takes on a life of its own. Original missions may become outdated but new missions are developed, not only with the intention of keeping the facility going, but often to actually enlarge it."

In Johnson's view, the bases themselves raise tensions that make wars likely, partly because other countries see them as threatening and offensive—for a host of reasons, from crimes committed with impunity against nationals by our personnel to religious and cultural dissonances between the hosts and their unwelcome guests. If the U.S. pursues its overblown career as an imperium, with the bases its most visible symbol and its means of extending power, what it can expect, he says, is "a state of perpetual war, leading to more terrorism against Americans wherever they may be and a growing reliance on weapons of mass destruction among smaller nations as they try to ward off the imperial juggernaut."