The Pentagon may command the most powerful military machine on earth, but it still has insufficient troops to comfortably carry out all of the missions assigned to it.
By Jonathan Marcus
BBC defence correspondent
It is combat power - fighting units - that matter, and these are in short supply.
Of the 31 combat brigades in the US army, some 18 are currently deployed, preparing for deployment, or have just returned from active duty overseas.
Many soldiers and their families in what is still an all-volunteer army are unhappy - and the latest efforts by the Pentagon to maintain unit cohesion by preventing troops from leaving the service while their units are deployed in Iraq or Afghanistan will only add to the pressures.
Tours of duty have had to be extended
That is why cash inducements are being offered to get troops to re-enlist.
Bear in mind that the US military is also undergoing a period of profound transformation, with some units adapting to new roles and new equipment.
It was not supposed to work like this.
Military transformation, which in some ways has only just begun, was intended to provide a lean US military that could fight and win quickly against almost any opponent.
That part worked in Iraq.
But the hope was that peacekeeping or security duties could then be handed over to other countries. Instead, the US has had to shoulder the bulk of the burden.
This means extending tours of duty, drawing more on reserves and getting the marine corps into the peacekeeping business.
The Pentagon used to have a planning guideline of being able to fight two medium-sized wars almost at the same time.
That now looks like fantasy as military planners cope with dealing with a sizeable commitment in Iraq that could last for years.