Imagine after the Battle of Fredericksburg, during the Civil War, if Jefferson Davis had telegrammed Gen. Robert E. Lee, saying: "You've been out there for a year now. Why don't you come back to Richmond?"

Or what if Gen. George S. Patton, after the North African and Italian campaigns in World War II, had been told that he had served sufficient time in the field and would be returning to Washington for an office assignment instead of going on to the Battle of the Bulge?

In generations past, when generals went off to lengthy wars, they tended to stay away for years. But consider the current case of Iraq. There, the Pentagon has adopted a policy of rotating generals about as frequently as it rotates troops, which means few have remained in the field longer than a year at a time.

Justifying the periodic changeover in top brass, defense officials cite a desire to keep commanders with the divisions and corps they lead and thus maintain unit cohesion. Other considerations play a part, too, officials say, including the grueling, unconventional nature of the conflict, a concern for sustaining a decent quality of life in today's all-volunteer military and the prospect that U.S. involvement in Iraq is going to last awhile and so will require a system for replenishing military leaders as well as their forces.

"It's a pretty hard existence," said one Army general who has served in Iraq. "It's 16, 17, 18 hours a day, seven days a week, nonstop in another culture, operating at 120 degrees heat on occasion.

"It's not only the troops we have to care about," he added. "It is senior leaders as well, and they do have kids and families."

Still, with the U.S. military occupation of Iraq now well into its third year, some defense experts in Congress and think tanks have challenged the turnover in senior military posts, warning that it has led to a loss of experienced leadership in the field.

"This is deeply unwise," Sen. John McCain (R-Ariz.) said in a speech earlier this month, adding his voice to those complaining about the rotation policy. "If these were the best men for the task, they should still be on the job.

"These generals and other senior officers build, in their time in Iraq, the on-the-ground and institutional knowledge necessary to approach this conflict with wisdom," he added. "We need these commanders and their hard-won experience to stay in place."

Such arguments for extending the tours of top officers come against the backdrop of mounting calls in Congress for withdrawing U.S. troops from Iraq altogether. But even if U.S. force levels start to decline next year as expected, generals probably will be needed for some time to lead those troops still in country.

Two generals who played key roles commanding U.S. troops during the rise of Iraq's insurgency in 2003 and 2004 have spent this year in Pentagon office jobs: Lt. Gen. James T. Conway, who headed Marine forces in western Iraq, now serves as chief operations officer on the Pentagon's Joint Staff; and Lt. Gen. Ray Odierno, who commanded the 4th Infantry Division overseeing north-central Iraq, is now assistant to the chairman of the Joint Chiefs of Staff.

But perhaps the most irksome case for McCain and other rotation critics is the recent reassignment of Army Lt. Gen. David Petraeus. In the 15 months Petraeus spent leading the development of Iraqi security forces, he is credited with reenergizing what had been a sputtering effort. He left Iraq in September for Fort Leavenworth, Kan., where he oversees the advanced training of Army officers, while in Iraq, many of the nation's fledgling security forces remain years away from being able to operate entirely on their own.

"I said to him, 'If you are the best, don't we owe it to our country and to our young men and women in uniform to have you still in Iraq?' " said Andrew F. Krepinevich, executive director of the Center for Strategic and Budgetary Assessments. "I said, 'The only reason we should bring you back is if you're burned out.' But he said he wasn't."

Petraeus declined to comment on the record for this report, but several defense officials said he was overdue to come home after spending nearly 21/2 years in Iraq (including time commanding the 101st Airborne Division in 2003-2004) and an earlier year-long stint in Bosnia.

Army Gen. John P. Abizaid, the commander of U.S. forces in the Persian Gulf region, is known to oppose having generals who lead divisions and corps stay longer than their units. But he and top Pentagon authorities have taken steps to ensure that a few seasoned officers remain longer in key staff positions and that several senior officers with experience in Iraq are brought back.

For instance, Army Gen. George W. Casey Jr., who has served as the top U.S. officer in Iraq for nearly 11/2 years, now plans to remain at least until summer. In line to become his senior tactical commander next year is Maj. Gen. Peter W. Chiarelli, who had charge of the Baghdad area last year as commander of the 1st Cavalry Division. And Lt. Gen. Martin Dempsey, who has replaced Petraeus, also spent an earlier tour commanding a U.S. division in the Baghdad area. Dempsey has been told to expect this time to stay as long as 11/2 years.

Army Lt. Gen. David Petraeus, who is credited with reenergizing the development of Iraqi security forces, is now back in the United States.