The announcement comes just days after President Obama announced that the US combat mission in Iraq would end by September 2010, but left open the question of when they would begin to return.
The redeployment could be seen as a down payment on the promised withdrawal of forces. Administration officials have indicated that, as expected, most of the rest of the combat forces won't return from Iraq until much later this year or early next, in order to ensure security for the Iraqi elections.
The Afghanistan mission also plays a role: The brigades that would have replaced the two returning from Iraq are being sent instead to Afghanistan, where Mr. Obama has promised more troops to deal with the deteriorating security.
Gen. Ray Odierno, the top US commander in Iraq, announced Sunday that over the next six months, two combat brigade teams and their supporting elements, or about 12,000 troops, would return home from Iraq without being replaced. An American F-16 fighter squadron will also be sent back without being replaced.
That will reduce the total combat force in Iraq from the current 14 brigades to a total of 12 brigades. General Odierno also formally announced that a British brigade of about 4,000 troops would be sent home without being replaced.
"The time and conditions are right for coalition forces to reduce the number of troops in Iraq," Odierno said in a prepared statement. "The successful provincial elections demonstrated the increased capability of the Iraqi army and police to provide security."
At the height of the surge of US troops in 2007, there were a total of 20 combat brigades deployed to Iraq, driving up the total number of American forces there to 166,000 in October 2007. When the surge ended last year, US commanders sent back five brigades, or about 20,000 troops.
The Pentagon then reduced American presence further by another brigade, leaving 14 combat brigades in Iraq and a total of 140,000 American troops.
"After we remove our combat brigades, our mission will change from combat to supporting the Iraqi government and its security forces, as they take the absolute lead in securing their country," Obama said during a speech at a Marine base in North Carolina Feb. 27.
Iraq's own security force has grown in recent years to more than 600,000 and become increasingly more capable.
The combination of US troops and Iraqi security forces has improved security in the country since the height of the insurgency in 2004. About 135 American troops were killed in April that year, according to icasualties.org.
Last month, 16 Americans were killed in Iraq, according to the website.
There remain pockets of insurgency in some areas, including in the north around Mosul and Baquba. US commanders are concerned that the drawdown of US forces is done carefully so as not to alter the security situation on the ground. Generally, US forces will pull out of the cities and back to more centralized bases before withdrawing altogether.
Although Obama pledged to end the combat mission in Iraq by September 2010, he has also said he will leave as many as 50,000 American troops in Iraq for another year or so after that to perform advisory and training roles.
The current agreement between Iraq and the US requires all US forces to be out of Iraq by the end of 2011. But experts and military commanders believe new agreements will ensure that some residual American forces will reside in Iraq even after that time.