The Pentagon has admitted it CAN NOT replace the 5 brigades that Bush sent to man the Surge in Iraq. The number of U.S. Army combat units needed to sustain the current troop levels in Iraq do not exist today in the Army.
The only options to come up with the 5 brigades to continue the Bush Surge beyond the spring on 2008 are as follows:
Use the National Guard
Extend tours in Iraq for Army units beyond the current 15 month tours.
Reduce the time out of Iraq below the current 12 months.
The Army said all of these options will damage the Army and or the National Guard. Short of immediately reestablishing the draft and quickly training another 5 Brigades between now and spring, the limiting factor to continue the Bush Surge is the lack of manpower in the U.S. Army. The Marines are in a similar situation and they can not make up the shortfall.