WASHINGTON – America is expanding its military force in Iraq to the highest level of the war – even higher than during the initial invasion in March 2003 – to bolster security in advance of the national elections coming up in January.

The 12,000-troop increase is to last only until March, but it says much about the strength and resiliency of an insurgency that American military planners did not foresee when Baghdad was toppled in April 2003.

Brigadier General David Rodriguez, deputy operations director of the Joint Staff, told reporters yesterday that the American force will expand from 138,000 troops today to about 150,000 by January.

The previous high for the American force in Iraq was 148,000 on May 1, 2003, when President Bush declared that major combat operations were over and most soldiers thought the war had been won. The initial invasion force included thousands of sailors on ships in the Persian Gulf and other waters, plus tens of thousands in Kuwait and other surrounding countries.

The expansion in Iraq will be achieved by sending about 1,500 troops from the 82nd Airborne Division in Fort Bragg, N.C., this month and by extending the combat tours of about 10,400 troops already in Iraq.

Defense Secretary Rumsfeld approved the moves yesterday, according to a Pentagon statement.

“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,” the statement said.

The Pentagon originally expected to train and equip enough Iraqi government forces to fill the security gap in the weeks leading up to the elections, but that hope was not fulfilled.

The military 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 the service. In this case, General George Casey, the most senior American commander in Iraq, decided it was necessary to keep up pressure on the insurgents while also 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 that they would remain until mid-January. Now they are being extended until mid-March.