GETTING out of a war requires as much planning as getting into one.

Last week, Defense Secretary Robert Gates gave the strongest indication yet that the Bush administration is thinking in detail about an eventual withdrawal. In a letter to Senator Hillary Clinton, Mr. Gates wrote: “You may rest assured that such planning is indeed taking place with my active involvement.”

Here are five questions that any administration — this one, or its successor — will have to answer as part of an exit from Iraq.

How Fast Can the Troops Leave?

Large numbers of American soldiers have left a modern war zone, but never so many from a still-hostile region.

Analysts agree that most would be airlifted, a far safer path than Iraqi roadways. (But many more soldiers would be needed to protect convoys of equipment that can only travel on the ground.)

“If you wanted to get all the troops out of Iraq, it’s just a question of how many airliners the Department of Defense can rent,” said Max Boot, a senior fellow at the Council on Foreign Relations. “In a pell-mell escape, it could probably be done in a matter of months, without taking into account any of the equipment.”

Perhaps, but rising chaos could jeopardize even a partial troop withdrawal.

“Planning is hard to do in a vacuum,” Mr. Boot said.

The United States Central Command, which runs the Iraq effort, is likely alone in its ability to create a detailed blueprint for withdrawal. Because this war has no precedent, plans envisioned outside of the Pentagon are only guesses, he said.

“All of this would be based on questionable assumptions, because we don’t have a lot of experience with this type of operation,” Mr. Boot said.

Can Departing Soldiers Be Shielded From Attack?

Troops concentrated in convoys that are transporting huge quantities of supplies out of Iraq make tempting targets. In the south, British forces have been attacked by militants as they pull back.

“We’re probably going to get stuck fighting our way out,” said Stephen Biddle, a senior fellow at the Council on Foreign Relations, especially in a rapid withdrawal. Mr. Biddle gives the current increase in troop levels a limited chance of successfully stabilizing the country, but it will take perhaps two years, with more casualties in the meantime. Hence the calculation: Withdraw with casualties now, or risk a better exit in a few years? How great is that risk?

Who Stays Behind?

There are up to 100,000 Iraqi contractors, perhaps more, working for the United States. After a pullout, many of them could be at risk from reprisals by anti-American forces.

“Do you move out Iraqis who have been working for us? How do you choose them?” said Anthony H. Cordesman, a military analyst at the Center for Strategic and International Studies. “The more ruthless you are about it, the quicker you can get out.”

So far, few Iraqis — either contractors or refugees — have been allowed to immigrate to the United States. The State Department has said thousands could potentially be resettled here in 2007. According to a bipartisan group of senators seeking special status for Iraqi refugees, only 63 have been admitted this year.

What to Take? What to Leave? What to Destroy?

After more than four years of buildup, the American footprint in Iraq is enormous. There are more than 75 major bases:

Some have their own retail stores, with products from magazines to luxury goods like large-screen televisions for purchase by soldiers. There are grocers, fitness clubs and fast-food outlets, in addition to the usual military infrastructure. Besides bases, there are hundreds of smaller sites for storage, ammunition and fuel.

The time required to salvage these multibillion-dollar installations and their contents could stretch the patience of Americans who want to see the war end soon. (In a July 20 New York Times poll, 66 percent of respondents wanted some or all of the troops out.)

But a quick exit has its own costs.

“The faster you move out, the more you have to leave behind or destroy,” said Mr. Cordesman of the Center for Strategic and International Studies. “There’s no ideal.”

Voters and political candidates, he says, are looking for a quick-exit “fantasy.”

“If you blow everything up, take the critical vehicles and get the people out, you can do it in a month. But why?”

With individual missiles costing $100,000 or more and armored Humvees about $380,000, the staggering value of materiél demands a longer, more complete withdrawal, Mr. Cordesman said. “You can leave an awful lot of things behind. But that borders on the insane.”

How Long to Repair and Ship Vital Equipment?

At the end of their duty, sophisticated combat aircraft aren’t simply loaded on to ships bound for the United States. They must be thoroughly washed of sand and contaminants until sterile, then shrinkwrapped to protect them from sea air.

“Everything has to be cleaned and pass an agricultural inspection,” said William G. Pagonis, a retired three-star Army general who directed logistics in the 1991 Persian Gulf war. “It’s not an easy task.”

Strict controls are meant to keep out any pathogens or insects that might threaten crops if carried to the United States.

Conditions in Iraq this time will undoubtedly be worse than during the much-lauded efforts by General Pagonis 16 years ago. Equipment, in use much longer, needs more extensive repairs. The threat of attacks will preoccupy soldiers, slowing repairs.

In 1991, the work was anticlimactic, with little attention from the press.

“Everyone wanted to cover the parades,” Mr. Pagonis said. “The media was not interested in the drawdown. It was a very rewarding time for logisticians.”