The AP has a good article that outlines the troop rotation math that shows why the twenty ground combat maneuver brigades are unsustainable:
Sapped by nearly six years of war, the U.S. Army has nearly exhausted its fighting force and its options if President George W. Bush decides to extend the Iraq buildup beyond next spring.
The Army's 38 available combat units are deployed, just returning home or already tapped to go to Iraq, Afghanistan or elsewhere, leaving no fresh troops to replace five extra brigades that Bush sent to Baghdad this year, according to interviews and military documents reviewed by The Associated Press.
That presents the Pentagon with several painful choices if the U.S. wants to maintain higher troop levels beyond the spring of 2008:
_Using National Guard units on an accelerated schedule.
_Breaking the military's pledge to keep soldiers in Iraq for no longer than 15 months.
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_Breaching a commitment to give soldiers a full year at home before sending them back to war.
For a war-fatigued nation and a Congress bent on bringing troops home, none of those is desirable.
In Iraq, there are 18 Army brigades, each with about 3,500 soldiers. At least 13 more brigades are scheduled to rotate in. Two others are in Afghanistan and two additional ones are set to rotate in there. Also, several other brigades either are set for a future deployment or are scattered around the globe.....
Casey said he would not be comfortable extending troops beyond their 15-month deployments. But other military officials acknowledge privately that option is on the table. [my emphasis]