General Odierno said the five additional brigades added this year under the president’s troop increase were likely to be withdrawn on a timeline parallel to their arrival in Iraq. Under this timeline, which is not yet the official plan, the troop increase would end by April with the five brigades leaving Iraq one each month, with American force levels returning to the troop levels existing before the increase by next August, he said.

Central to the internal debate on a “postsurge” strategy is the extent to which American troops would be able to ask Iraqi forces to take the lead on security missions in critical sections of the country, particularly in Baghdad. Many Democrats in Congress, and even some Republicans, have demanded that Americans hand over more security missions to the Iraqis.

Although no decision has been made about the full extent of the American combat mission next year, administration officials and military officers say the troops in Iraq would shift priorities to training and supporting Iraq forces. They said the large contingent of Special Operations forces now in Iraq would continue missions to capture and kill terrorist and insurgent leaders, and to disrupt their networks.

Under the new strategy, administration and military officials say, some American troops would be withdrawn from relatively stable regions in the Kurdish north and Anbar Province, and could be shifted to still-contested areas or into noncombat missions. But the officials say they expect the strategy to call for American forces to retain a leading role at least well into next year in the dangerous fight to maintain security in Baghdad and a strategic ring of communities in a band around the capital.

“That’s the center of gravity,” one official involved in the strategy discussions said.

Military officials said General Petraeus was still revising his calculations on what the exact mission of American troops should be, and how many would be required to carry it out. One senior administration official said the political debate focused too much on the overall number of Americans in Iraq. “It’s more than just the raw number of troops,” the official said. “It’s where they’re deployed and how.”

In a preview of the September report to assess progress, the administration said in July that accomplishments were satisfactory in nearly half of the 18 benchmarks set by Congress.

In recent weeks, several military, diplomatic and administration officials have sought to change the focus of the review, saying the official benchmarks might not be the best measures of success in Iraq, as they do not weigh factors like the growing power of local leaders and the willingness of the population to demand political reconciliation in the central government.