The two combat brigades that would have replaced them will go to Afghanistan instead.
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."