The force will grow from 138,000 to about 150,000 by mid-January, the Pentagon said Wednesday.
Extra troops are needed to bolster security before the national elections scheduled for Jan. 30. The increase in troop strength also underscores the fact that, despite enormous effort and cost, American commanders have yet to train and equip enough Iraqis for security duty.
Lt. Gen. Lance Smith, deputy commander of Central Command, which is responsible for U.S. military operations throughout the Middle East, told reporters at the Pentagon last month that the insurgents have managed to intimidate many Iraqis into not cooperating with the Americans.
The expansion of the U.S. force also recalls assertions made by some Bush administration officials when the invasion was launched that although stabilizing the country would not be easy or cheap, it certainly would not require more U.S. troops than it took to topple Baghdad.
In other developments:
As it turns out, the post-invasion period has been far costlier in blood and treasure than almost anyone predicted. When Mr. Bush declared major combat operations were over May 1, 2003, the United States had about 148,000 troops in Iraq — slightly more than when the war began two months earlier and more than were there when Baghdad fell in early April.
The Pentagon said Wednesday that Defense Secretary Donald H. Rumsfeld approved a plan to send 1,500 soldiers of the 82nd Airborne Division from Fort Bragg, N.C., to Iraq this month and to extend by 60 days the combat tours of about 10,400 soldiers and Marines in Iraq who were to come home in January.
Most of those whose tours are being extended will serve two months longer than the 12-month tours the Army set as a standard limit to avoid putting too much stress on troops and their families.
The 12,000-troop increase is to last only until March, but it says much about the strength and resiliency of an insurgency that U.S. military planners did not foresee even a year ago, when they were focused on capturing deposed Iraqi President Saddam Hussein.
Pentagon officials said they preferred to expand the force in Iraq mainly by keeping some troops there longer rather than sending thousands of fresh troops from the United States.
"They are the most experienced and best-qualified forces to sustain the momentum of post-Fallujah operations and to provide for additional security for the upcoming elections," a Pentagon statement said.
The military normally is reluctant to extend soldiers' combat tours because of the potential negative effect it could have on their families, and thus on their willingness to remain in uniform. In this case, Gen. George Casey, the most senior U.S. commander in Iraq, decided it was necessary to keep up pressure on the insurgents while providing security for the elections.
One unit, the 2nd Brigade of the 1st Cavalry Division, is being extended for the second time. Its soldiers originally were told they would be going home in November at the end of a 10-month assignment, but in October they got the news they would remain until mid-January. Now they are being extended until mid-March.
Rumsfeld's decision also applies to:
About 4,400 troops of the 2nd Brigade of the 25th Infantry Division, which is operating in north-central Iraq. They will stay until mid-March, instead of departing in early January. Those soldiers' home bases are mostly in Hawaii.
About 2,300 members of the 31st Marine Expeditionary Unit, based in Okinawa, Japan, Hawaii and California, who will stay until mid-March instead of leaving in January.
About 160 soldiers of the 66th Transportation Company, based in Germany. They were due to depart Iraq in early January but instead will stay until early March.
The Army generally relies upon the 82nd Airborne to keep one of its three brigades on short-notice alert year-round to deploy abroad if there is a crisis. Shortly before the October elections in Afghanistan, about 600 members of the 82nd Airborne were sent there to strengthen security.
Sen. Jack Reed, D-R.I., a critic of the administration's handling of the war, said the Pentagon's announcement confirmed that the effort to stabilize Iraq would take years, with no certainty of success.
"This announcement makes it clear that commanders in Iraq need more troops and that this will be a long and very expensive process for the United States," Reed said. "It is still not clear whether Iraq will emerge from this chronic violence as a viable and stable country."