WASHINGTON - In a sign of the Iraq war's strain on the U.S. military, the Army is planning to send into combat thousands of soldiers whose normal job is to play the role of the "enemy" at training ranges in California and Louisiana, defense officials said Tuesday.

The Pentagon also is considering adding yet another National Guard brigade, the 155th Separate Armored Brigade from Mississippi, to the mix of active-duty and reserve units designated for the next rotation of ground forces into Iraq this year and in early 2005, other Army officials said.

With nearly every other major combat unit either committed to or just returned from Iraq or Afghanistan, the Army is planning to call on two battalions and one engineer company - about 2,500 soldiers - from the 11th Armored Cavalry Regiment, which serves as a professional enemy force at the National Training Center at Fort Irwin, Calif. The regiment last saw combat in the Vietnam War.

The Army boasts of the "tough and uncompromising standards" of the 11th Armored Cavalry, which it says makes it the premier maneuver unit in the Army and "the yardstick against which the rest of the Army measures itself."

Similarly, the 1st Battalion of the 509th Infantry, which acts as the Opfor, or opposition force, for light infantry and special operations training at Fort Polk, La., is being called to Iraq, according to two Army officials who discussed the matter on condition of anonymity.

The 509th Infantry has not seen combat since World War II, although five members of the unit served as "pathfinders," or advance scouts, during the 1991 Gulf War; two were killed and one was taken prisoner.

Both the National Training Center and Fort Polk's Joint Readiness Training Center will remain open, the officials said, with National Guard soldiers expected to fill in for the units going to Iraq.

The Navy said Tuesday that it is sending a second aircraft carrier, the USS John C. Stennis, into the western Pacific, apparently to compensate in part for the planned deployment to Iraq this summer of an Army combat brigade based in South Korea.

The Stennis, which left its San Diego home port Monday, will participate in an exercise off Alaska in June and then join the USS Kitty Hawk, which is permanently based in Japan, in the western Pacific.

The next U.S. troop rotation in Iraq will kick off this summer, not long after the June 30 turnover of partial political control to an interim Iraqi government and a coinciding change in the U.S. military command structure in Iraq.

Lt. Gen. Ricardo Sanchez, who has been the top commander in Iraq since May 2003, is to be replaced this summer by a four-star general, most likely Gen. George W. Casey, officials said.

The move is part of a restructuring of the U.S. command in Iraq. The idea is to have a four-star there to focus on the bigger picture, including working with U.S. and Iraqi political authorities, while a lieutenant general handles the day-to-day command of combat.

Although the Abu Ghraib prisoner abuse happened on his watch, Sanchez's departure is not related to that, said Larry Di Rita, chief spokesman for Defense Secretary Donald H. Rumsfeld. Any suggestions to the contrary, he said, are "just wrong." Sanchez has testified to Congress that he was not aware of the abuse until it was reported to him in January.

Rumsfeld's original plan was to replace Sanchez this summer with Rumsfeld's senior military assistant, Lt. Gen. Bantz Craddock, and to nominate Sanchez for a fourth star and the command of U.S. Southern Command, a senior defense official said. Because of the prisoner abuse controversy, Rumsfeld decided it would take too long to get Sanchez confirmed by the Senate, the official said, and so decided on the noncontroversial Casey for the Iraq post, probably leaving Craddock for the Southern Command job.

Brig. Gen. Mark Kimmitt, spokesman for the U.S. military in Baghdad, told reporters Tuesday that Sanchez's 5th Corps headquarters, of which he is the commander, left Iraq in February.

"We have always expected Gen. Sanchez to depart some time after the transfer of sovereignty," Kimmitt said. "My personal expectation was, like me, he would be departing in the July time period."

Casey would be an unusual choice for the top military post in Iraq, in part because he has served for less than a year in his present position as vice chief of staff, the No. 2 staff job in the Army.

In his 33-year Army career, Casey has never served in combat. During the final years of the Vietnam War he served with the 509th Infantry, based in Germany and later in Italy, and during the 1991 Gulf War he was in the Pentagon as special assistant to the Army chief of staff.

Prior to becoming the Army's vice chief of staff last October, Casey served as director of the Joint Staff.

President Bush praised Sanchez during an appearance before reporters in the Oval Office. "Rick Sanchez has done a fabulous job," he said as he met with a group of Iraqis. "He's been there for a long time. His service has been exemplary."