WASHINGTON - The military has begun a rotation of forces in Iraq and Afghanistan that amounts to the largest movement of American troops in decades, Army officials said Thursday.

The changes present an enormous logistics challenge not only for the Army and Marine Corps, but also for the Air Force and Navy, whose planes and ships are ferrying the troops to and from Iraq.

They also add to the security concerns of commanders on the ground in Iraq, who worry that fighters opposed to the U.S. occupation will look for security gaps created by the monthslong rotation.

The transition began this week with the return from Iraq of the first 200 members of the 101st Airborne Division to their home base at Fort Campbell, Ky., and the departure from Fort Bragg, N.C., of paratroopers of the 504th Parachute Infantry Regiment of the 82nd Airborne. Ships carrying equipment for the 25th Infantry Division left Hawaii this week, officials said.

In a related matter, Army officials said a military team has arrived in Iraq to advise ground commanders on ways to cope with the homemade bombs that insurgents have detonated along roads used by U.S. convoys over the past several months, often killing or maiming soldiers.

The team also is doing forensic studies of actual detonated explosives and consulting with commanders on potential improved technical means of defeating these low-tech, improvised bombs.

The Army, working with the other military services, has been planning the rotation of forces in Iraq for many months. The Pentagon originally hoped that some of the troops there now could be replaced by international forces, but few have been offered for the dangerous duty.

The approximately 130,000 U.S. troops in Iraq now will be heading home over the coming four months, to be replaced by a more mobile, less heavily armed force of about 110,000. That force will include about 20,000 Marines and an increased proportion of National Guard and Reserve troops.

A senior Army official provided details of the rotation Thursday on condition of anonymity. He said the movement would involve the equivalent of 81/2 of the Army's 10 active-duty divisions. The Army also has troops committed in South Korea, Germany, the Balkans and elsewhere.

It also includes Marines of the 1st Marine Expeditionary Force from Camp Pendleton, Calif., and the 3rd Marine Aircraft Wing at Miramar, Calif. - some of whom participated in the invasion of Iraq last March and the dash to Baghdad. Marines left Iraq late last summer; they are being called on again because the Army is too stretched to meet all the troop requirements.

The Navy's 3rd Fleet announced on Wednesday that the helicopter carrier USS Boxer will head for the Persian Gulf next week from San Diego with equipment for the 1st Marine Expeditionary Force. The ship will carry 200 Marines; the rest of them will fly there in a few months.

The Marines are to operate in the area west of Baghdad, replacing soldiers of the 82nd Airborne, the 1st Infantry Division and the 2nd Light Cavalry Regiment. That area includes Fallujah, Ar Ramadi and other cities that represent a stronghold of the anti-American insurgency.

Two brigades of the 1st Infantry, along with a brigade of the 25th Infantry and the 30th Infantry Brigade of the North Carolina National Guard, will operate in the area around Tikrit and Kirkuk in northern Iraq, replacing the 4th Infantry Division and the 173rd Airborne Brigade.

A brigade of the 2nd Infantry Division, known as the Stryker Brigade, will operate in the Mosul area of northern Iraq where the 101st Airborne has had its headquarters. The 101st is one of the last major Army units left in Iraq that also participated in the initial invasion and defeat of Baghdad. The Stryker Brigade is already in Iraq and moving north to Mosul.

The 1st Cavalry Division from Fort Hood, Texas, will replace the 1st Armored Division in Baghdad. Working with the 1st Cavalry will be the 39th Infantry Brigade of the Arkansas National Guard.