The Army, the largest of the armed services, has portions of every major active-duty combat unit committed to either Iraq or Afghanistan, with the exception of the 2nd Infantry Division, which is in Korea.

The rotation plan, approved by Defense Secretary Donald Rumsfeld, has been weeks in the making amid angst in the ranks of units like the 3rd Infantry Division, whose soldiers had expected to go home shortly after they stormed Baghdad in April and ousted Saddam Hussein's government.

Waiting for the Pentagon to figure out a rotation plan has been made more difficult by the daily attacks that U.S. soldiers are facing in Iraq; 97 U.S. troops have died in Iraq since President Bush declared major combat over on May 1.

Gen. John Keane, the acting Army chief of staff, told a Pentagon news conference that because the 3rd Infantry was first to deploy to the Persian Gulf in the fall in preparation for the war, its members will be the first to be relieved. Elements of the 82nd Airborne Division will replace them in September. One of the 3rd Infantry's three brigades already has left Iraq.

Also due to depart in September is the 1st Marine Expeditionary Force, including the 1st Marine Division. They will be replaced by an international division commanded by Polish forces.

Together, the 3rd Infantry and the 1st Marine Division captured Baghdad in early April. The Marines then moved north to Saddam's hometown of Tikrit, and they are now deployed in south-central Iraq.

Next in line to go home is the 2nd Brigade of the 82nd Airborne Division, which will depart in January but not be replaced. The only other combat force that will go home without being replaced is the 173rd Airborne Brigade, which will return to its base in Vicenzia, Italy, next April.

Keane acknowledged that the requirement for troops in Iraq over the coming months puts a strain on the Army.

"Certainly the force is stretched," he said. But he also said morale is high because soldiers believe they are making important contributions to defending America against terrorists.

Keane also said that while he has yet to decide whether to ask Rumsfeld to expand the size of the active-duty Army, he believes it has critical shortages in some areas.

"We need more infantry, we need more military police, we need more civil affairs (specialists)," he said. "Those are facts."

The units that go to Iraq next will serve one-year tours, Keane said, with a few exceptions. One exception is the National Guard brigades, which will serve six-month tours. Also, the newly created Stryker Brigade, based at Fort Lewis, Wash., will be there for six months. It was created out of troops and equipment from the 3rd Brigade of the 2nd Infantry Division.

Keane said he was confident the Stryker Brigade, which uses wheeled vehicles instead of tanks for its armor and is intended to be a model for future Army combat forces, is ready for action in Iraq even though it has yet to be formally certified by the Army as "operationally" ready.

"We have put them through their paces and they are ready to go," he said, referring to the Stryker Brigade's recent work at the National Training Center in California and the Joint Readiness Training Center in Louisiana.

Keane said the Army had not yet decided which National Guard brigades would serve, but a decision was likely by Friday.

The rest of the rotation plan:

• In March and April, the 1st Infantry Division, together with a National Guard brigade, will relieve the 4th Infantry Division, which will have been in Iraq for one year.

• In February, March and April, elements of the 1st Cavalry Division, with a National Guard brigade, will replace the 1st Armored Division.

• In March and April, a brigade of the 1st Cavalry will replace the 2nd Armored Cavalry Regiment.

• In October, the Stryker Brigade will deploy and operate with the 3rd Armored Cavalry Regiment, which is already there. They will operate together until March or April.

• In February and March, the 101st Airborne Division will be relieved by a multinational division yet to be constituted.