The rest of the boost will come from sending a few brigades earlier than planned and extending the tours of others. Affected will be units based in Minnesota, Kansas, Georgia and Washington, said a military official who spoke on condition of anonymity because the information had not been publicly released.
Whether the military push proves successful or not, it will have ramifications later for an Army and Marine Corps that already are stretched thin.
Some units will have less time at home for rest and retraining between tours than their commanders would like. The faster pace of deployments also could force the Pentagon to call on National Guard and Reserve units more frequently — possibly to remobilize some that already have served in Iraq.
According to the military official, who provided no dates,
In addition, the 2nd Brigade of the 82nd Airborne, which had not been scheduled to go to Iraq this year, is expected to move into Iraq by mid-January. The brigade's home base is Fort Bragg, N.C.
Brigades typically have about 3,500 troops.
President Bush planned to visit Fort Benning on Thursday morning as part of an effort to promote his revamped Iraq strategy.
The Pentagon made no official announcement about its troop-boost plan.
White House officials said an extra 17,500 Army soldiers would be sent to Baghdad and the Marines would have an extra 4,000 troops in western Anbar Province, the heart of the Sunni Arab insurgency. Some will be placed inside Iraqi Army units to accelerate their training. Others will support Iraqi Army units that will be expected to do the bulk of the street patrols and other missions to quell Baghdad's sectarian violence.
The increase is to be achieved over a period of months. At its height, the troop total — now at about 132,000 — apparently would reach about 153,500, although the Pentagon did not release a timeline for the increases.
That is not appreciably higher than just two months ago, when it stood at 152,000. And it is a little less than the 160,000 level of one year ago, in the immediate aftermath of Iraqi elections.
Even so, it marks a major change of direction for a Pentagon which last summer believed it could reduce U.S. troop levels to below 100,000 by now. Instead it built up forces in the summer and fall as sectarian violence escalated.
Kevin Ryan, a retired Army brigadier general at Harvard University's Belfer Center for Science and International Affairs, said the impact of this troop increase on the overall health of the Army will depend in part on how long it is maintained.
If it is only for a few months, as appeared possible from early indications of Army and Marine Corps planning, then "we won't have long-term damage to the force" from excessive strain, Ryan said in a telephone interview.
In congressional testimony in November, Gen. John Abizaid, the top commander for U.S. forces in the Middle East, said a troop increase of 20,000 could not be sustained for long because the Army and Marine Corps simply are too small.
The Marines plan to extend the tours of two battalions in Anbar province by 60 days starting in February, one official said, speaking on condition of anonymity. That would achieve a net gain of 4,000 troops because the two battalions would remain after their designated replacements arrived from the United States in February.