"The security nationwide in Iraq will be a combination of coalition forces and the new Iraqi government's re-established police forces and armed forces," Gen. Peter Pace, the vice chairman of the Joint Chiefs of Staff, said in an interview today.

But Paul D. Wolfowitz, the deputy defense secretary, said on Sunday that it might take the United States more than six months to cede power to an Iraqi-led civilian authority.

The size and scope of any postwar security force has already stirred debate on Capitol Hill and at the Pentagon. There are more than 125,000 allied troops in Iraq now, with more than 100,000 Army troops including the Fourth Infantry Division, First Armored Division and First Cavalry Division moving into the region or on the way from the United States and Europe.

But Gen. Eric K. Shinseki, the Army chief of staff, has said several hundred thousand troops will be needed to keep the peace in postwar Iraq. Mr. Wolfowitz dismissed General Shinseki's assessment as "wildly off the mark." Pentagon officials have put the figure closer to 100,000 troops.

General Shinseki, who is retiring in June, has repeatedly clashed with Defense Secretary Donald H. Rumsfeld, but the general's supporters cite his experience as a former commander of American peacekeeping forces in Bosnia, and warn that Pentagon officials have underestimated the job ahead of them.

"I don't think they understand the scope of the problem, but I think they're starting to see it right now with the chaos, looting, revenge killing and political intrigue," said William L. Nash, a retired Army major general whose brigade stayed in southern Iraq more than two months after the gulf war (news - web sites) in 1991.

"We are extraordinarily vulnerable from a force-protection standpoint as the cop on the beat," said one senior retired general, who voiced specific concern about the Iraqi capital. "There must be urgent consideration to have the Baghdad police do that job."

Pentagon officials say they have learned lessons from peacekeeping or security missions in Somalia, Haiti, Bosnia, Kosovo and, most recently, Afghanistan (news - web sites). But the operation in Iraq will be dominated by the United States and its allies in the war, not the United Nations (news - web sites), senior Pentagon officials said.

And unlike Afghanistan, where there were multiple reconstruction efforts, "We know we want something a little more corporate and more efficient with cleaner lines of authority and responsibility," said one senior Pentagon official familiar with postwar planning for Iraq.

Senior defense officials said today that the cost to police Iraq after the war could reach hundreds of millions, or even billions, of dollars, but it is unclear where the money would come from.

Any American financing, either through the State Department or Defense Department, would require new appropriations by Congress. But Bush administration officials are hoping that allies, frozen Iraqi funds and even future Iraqi oil proceeds might defray future security costs.

Any plans for postwar security will need to be flexible, Pentagon officials acknowledged today.

According to a senior Pentagon official, there are three roughly drawn phases for securing Iraq.

First, the American military, under General Franks or a deputy, will maintain a security force officials describe as "robust" to root out pockets of resistance and any guerrilla attacks, and provide a security umbrella to allow aid to flow freely in the country.

The Pentagon hopes to find allies to help with this job. At least a dozen nations have offered to help with reconstruction, but for most that does not yet include a security role.

Prime Minister Jean Chrétien of Canada said today that Canada, a mainstay in past peacekeeping missions, was "ready to help as soon as possible" in Iraq's postwar reconstruction. But an embassy spokesman here said it was too early to talk about whether Canada could contribute to a postwar security force.

Senator John W. Warner, a Virginia Republican who heads the Armed Services Committee, suggested today that NATO (news - web sites) might help with postwar security, and will hold a hearing Thursday to explore the idea.

The second phase will be to restore Iraqi civilian police forces back to their duties as quickly as possible. International law enforcement advisers will train the police, the senior Pentagon official said.

A retired Army lieutenant general, Jay Garner, is preparing to begin the Pentagon reconstruction and relief effort in Iraq, and will likely play a role in helping to establish a new civilian police administration, the senior Pentagon official said.

Finally, a force of gendarmes or constabulary drawn from allied countries' state police or national guard would patrol highways and perform riot control duties, the Pentagon official said.

"The military piece will be in place from Day One, and the other pieces will fall in behind," the senior Pentagon official said.

Some experts caution that because of its size and diverse religious and ethnic makeup, Iraq poses the most formidable set of security issues the military has yet faced.

"It's going to be a great challenge," said Gen. George A. Joulwan, a retired Army officer. "It's not only nation building for Iraq, but security building for the U.S."