Among the most likely conflicts the Pentagon foresees in the near term are withNorth Korea and Iran, the two remaining members of Bush's "axis of evil." The Bush administration accuses both of having ambitions to become a nuclear power; North Korea has already claimed it has nuclear weapons.

The U.S. military has timelines in place for defeating its potential adversaries, given enough soldiers, tanks, aircraft and warships to do the job. But with so much of those resources tied up fighting insurgencies in Iraq and Afghanistan, those timelines could slip, Myers said, according to the defense official.

About 138,000 American troops are in Iraq, according to U.S. Central Command. Another 18,000 are in Afghanistan.

Military officials have given no precise estimate when they will be able to significantly draw down the number of U.S. troops in Iraq, but some generals have suggested it could come next year if Iraqi security forces continue to improve in quality and grow in numbers.