Washington ? The National Guard and Reserve will take on more of the combat burden in Iraq next year, replacing some Army troops with a smaller, lighter and more mobile force equipped with fewer tanks and more Humvees.

Nearly 40 percent of the American forces in Iraq will be from the National Guard and Reserve after the Pentagon completes a massive switchout of troops starting in January — up from about 20 percent now.

Three National Guard infantry brigades will go, at least two of them slated for combat duties.

Overall, the Pentagon’s plan for replacing the 130,000 American troops in Iraq with a fresh contingent will shrink the force by 20 percent and result in a more mobile force, perhaps better suited to the guerrilla war that has been taking a sobering toll in U.S. deaths and injuries.

Stryker Brigade

The first changes will be seen even before the newly designated replacement force gets there. A contingent of 5,000 soldiers in a combat team called the Stryker Brigade, from Fort Lewis, Wash., is training in Kuwait in preparation for duty in Iraq. They are equipped with a new, speedier, lightly armored troop carrier and sophisticated communications tools to enable soldiers to locate guerrilla threats.

The Stryker Brigade is likely to see action in the so-called Sunni Triangle, the area between Baghdad, Ramadi and Tikrit where the resistance to U.S. forces has been deadliest.

“It is absolutely optimized for this kind of fight,” said Lt. Gen. Richard Cody, the Army’s deputy chief of staff for operations, who oversees the Army’s provision of fresh forces.

Nearly 40 percent of the 105,000 troops in the new force will be National Guard and Reserve after the switchout ends in April. That compares with about a 20 percent share in the current force of 130,000 troops.

And it won’t be just Army reservists; the Marines plan to use about 6,000 of their citizen-soldiers.

Groomed for speed

The main replacement force will arrive over a period of about four months, from January through April. They will be lighter and more agile than the units they replace; they will have two-thirds fewer tanks and Bradley armored troop carriers, trading firepower for mobility.

An armored division like the 1st Cavalry Division will equip two of the three battalions in each of its brigades with Humvee utility vehicles instead of tanks and Bradleys. The 1st Cavalry, based at Fort Hood, Texas, will actually be larger than a normal division, since it will operate with the 39th Infantry Brigade of the Arkansas National Guard.

The switch away from heavy armored forces has created such demand for Humvees that the Army is pulling every available one — fortified with add-on armor — out of the United States and Europe, Cody said.

Stop-loss

Not just vehicles are in heavy demand. The Army is so stretched for soldiers that it is imposing “stop-loss” on all units designated for duty in Iraq, Kuwait and Afghanistan — meaning those troops cannot leave the service even if they planned to retire. The clamp will remain during their duty in Iraq and three months beyond, Cody said in a recent interview.

The 1st Cavalry is likely to be given responsibility for the Baghdad area, replacing the 1st Armored Division.

The 1st Infantry Division, coming from several locations in Germany, will be joined by the 30th Infantry Brigade of the North Carolina National Guard. They are likely to operate in place of the 4th Infantry and 101st Airborne divisions in northern Iraq, including the Kurdish area.

The Bush administration had counted on getting a multinational division to replace the 101st Airborne, but that has not panned out. Multinational divisions led by Britain and Poland will continue operating in the less volatile south-central and southeastern parts of Iraq.

Elements of the 1st Marine Division, joined by one active-duty Army brigade, are expected to be assigned to western Iraq, including the Fallujah area, which has especially hostile to U.S. forces.