The Pentagon plans, announced Thursday, also include alerting an additional 43,000 National Guard and Reserve support troops that they may be sent to Iraq as well.
Under the rotation plan, the overall number of American troops in Iraq will actually fall to 105,000 by May from the current 131,600, senior officials said.
That net reduction in U.S. forces contrasts sharply with calls from some in Congress for increased troop strength. The Bush administration says it can fight the ongoing anti-occupation violence in Iraq with fewer U.S. forces because it is rapidly increasing the number of Iraqis trained for security missions.
In an added twist, the Army announced that soldiers in every unit designated for deployment to Iraq next year -- whether active duty or reserve -- will be prohibited from leaving the service during a period beginning 90 days before their departure to 90 days after they return.
That measure, known in the military as ''stop-loss,'' does not apply to the Marine Corps, which said it will dispatch about 20,000 Marines to replace the Army's 82nd Airborne Division in western Iraq, including the Fallujah area where attacks have been most numerous.
Lt. Gen. Jan C. Huly, deputy commandant of the Marine Corps for plans and operations, told a news conference that the Marines would spend seven months in Iraq, then be replaced by another 20,000-Marine contingent for seven months.
They will come from the 1st Marine Division, based at Camp Pendleton, Calif., which helped spearhead the invasion of Iraq last spring.
The Army will send the equivalent of three combat divisions to replace the four now in Iraq.
The rotation, combined with a switchout of troops in Afghanistan -- the 25th Infantry Division replacing the 10th Mountain Division in April -- is the largest sequence of troop movements for the Army since World War II, Lt. Gen. Richard Cody said in an interview. He is the Army's deputy chief of staff for operations.
The 1st Infantry Division will go from Germany, the 1st Cavalry Division from Fort Hood, Texas, a brigade of the 2nd Infantry Division from Fort Lewis, Wash., and a brigade from the 25th Infantry Division.
National Guard infantry brigades will be attached to both the 1st Infantry and 1st Cavalry.
Those units will replace the 82nd Airborne, the 1st Armored Division, the 4th Infantry Division and the 101st Airborne Division.
First to depart Iraq will be the 101st Airborne, which is the only one of the four divisions there now that participated in the drive to Baghdad last spring.
The net result: 20 percent fewer U.S. troops will be in Iraq by May. Defense Secretary Donald H. Rumsfeld told the news conference he is assuming the two multinational divisions now in Iraq -- led by Britain and Poland and totaling about 24,000 troops -- will remain through next year.
The Pentagon had been counting on a third multinational division, possibly led by Turkey, but that has not materialized.
The Bush administration has set no timetable for withdrawing American forces from Iraq. President Bush delivered a message to the troops on Thursday via the American Forces Radio and Television Service.
''Our mission in Iraq goes on and the war on terror is far from finished,'' he said, according to a transcript released by the Pentagon. ''The road ahead is difficult and dangerous, but I have complete confidence in you.''
Some of the troops rotating into Iraq will be returning for their second tour of duty there -- and some only a short time after they were sent home, Rumsfeld said.
Reservists will be called up for a maximum of 18 months, with a year in Iraq, Rumsfeld said.