The US Marine Corps says it has been authorised by President Bush to recall thousands of inactive reservists to serve in Iraq and Afghanistan.
US marines serve in some of the most dangerous parts of Iraq
Up to 2,500 of a pool of nearly 60,000 marines who have left active duty could be recalled to serve at any one time.
Thousands of marine reserves have already served in Iraq, but they were active reservists who train regularly.
Now inactive reservists obliged only to report one day a year may be recalled - against their will if necessary.
The Marine Corps describes the move as prudent planning, but critics will seize on the announcement as evidence the US military is overstretched in Iraq, reports the BBC's James Westhead in Washington.
The call-up, authorised by President George W Bush, will affect members of the Individual Ready Reserve (IRR).
Under a general contract, a marine serves four years on active duty, and then a further four years in the reserve forces. For this final four years, they can either elect to join the regular reserves - where they are paid and train regularly - or choose to join the IRR.
Col Guy Stratton, head of the Corps' manpower mobilisation section, told the news agency Reuters that in recent times fewer marines had been choosing to volunteer for the active reserves.
No expiry date
The Marine Corps have now been given permission to recall up to 2,500 marines for duty at a time.
But the authorisation is open-ended and will only expire once the so-called "Global War on Terror" (GWOT) - a war whose parameters are largely undefined - has ended, so many thousands could eventually end up serving.
Reservists could be required to serve up to 24 months, but will more likely serve between 12 and 18 months, said Col Stratton.
Our correspondent says the US army has already recalled around 10,000 inactive reserve soldiers.
But he says the smaller marine force plays a key role in Iraq, shouldering a significant portion of the fighting. They also serve in some of the most dangerous parts of the country.