The top US general, Richard Myers, has warned that ongoing military operations in Iraq and Afghanistan could limit the ability of the US to fight another war.
Demands on the US military in Iraq are heavy
Current commitments can be met, but a new conflict could take longer and cause more casualties, the chairman of the Joint Chiefs of Staff said.
He expressed his views in a classified report to Congress, parts of which have been leaked to the media.
There are about 130,000 US troops in Iraq and some 16,000 in Afghanistan.
The US Congress is currently considering whether to approve a further $82bn to fund the operations there.
'We will prevail'
The ongoing US military operations in Iraq and Afghanistan are placing heavy demands on personnel and equipment.
Gen Myers indicated to journalists that if the military were called upon to fight another war then current expectations regarding how long it would take to win that war might have to be revised.
But he said the US military was able to meet all of its current commitments.
"We will prevail," he said. "The timelines [to winning a new war] may have to be extended and we may have to use additional resources, but that doesn't matter because we're going to be successful in the end."
The document reportedly says that future conflicts may result in longer campaigns, higher casualties and greater collateral damage.
Independent US military analysts say the US could theoretically fight another war in Iran, say, or on the Korean peninsula, but it would place massive stress on the armed forces, especially the army.