The Bush administration has settled on a plan for a possible invasion of Iraq that envisions seizing most of the country quickly and encircling Baghdad, but assumes that Saddam Hussein will probably fall from power before U.S. forces enter the capital, senior U.S. military officials said.

Hedging its bets, the Pentagon is also preparing for the possibility of prolonged fighting in and around Baghdad. Administration war planners expect that, even if the Iraqi president is deposed from power, there could be messy skirmishes there and in Hussein's hometown of Tikrit, the military officials said.

The war plan, sometimes the subject of bitter arguments between senior civilian and military officials, has been refined in recent weeks even as the Bush administration pursued a successful diplomatic effort to secure a new U.N. weapons inspection system for Iraq. Officials said that the plan could still change in important ways, such as the precise number of troops required, but that the broad outlines are now agreed upon within the administration. Military officials said they will be prepared to go to war if Iraq flouts the new resolution, approved on Friday by the U.N. Security Council.

Most notably, the emerging U.S. approach tries to take into account regional sensitivities by attempting to inflict the minimum amount of damage deemed necessary to achieve the U.S. goals in a war. The plan aims to do that mainly by attacking quickly but with a relatively small force conducting focused attacks. But it also hedges by putting enough combat forces in the area -- including around 150,000 U.S. and allied ground troops -- to engage in close combat with the Special Republican Guard if Iraqi resistance is stiffer than expected.

"The point is that if things don't go as we hope, there will be enough forces on hand to deal with it," said one Defense Department official who was briefed on the plan late last month.

The dual nature of the U.S. war plan is designed to encourage Iraqis to revolt against Hussein. As an administration official put it in a recent interview, the plan aims to "create the conditions" under which Iraqis can do that. "I think ultimately this is more of a revolution that's going to happen, rather than something brought about by U.S. military power," he said.

To create those conditions, the U.S. invasion would begin with a series of simultaneous air and ground actions and psychological warfare operations, all aimed at destroying the security police and other institutions that help Hussein hold on to power. "You have to shake the regime to its core," said one knowledgeable defense expert. "You've got to pursue the pillars of the regime across the board."

Under the concept of operations briefed this fall to President Bush, rather than begin with a lengthy air campaign, as in the 1991 Persian Gulf War, an invasion would begin with the U.S. military swiftly seizing the northern, western and southern sectors of Iraq while launching airstrikes and other attacks on "regime targets" -- mainly security forces and suspected repositories of chemical and biological weapons -- in the remaining part of the country around Baghdad, military officials said.

Simultaneously, a nationwide "psychological operations" campaign that is already underway would use leaflets and radio broadcasts to try to persuade the Iraqi military to change sides and to tell the Iraqi population that they aren't being targeted. Also, troops and civilian officials would be warned against carrying out orders to use chemical or biological weapons.

If Hussein falls quickly, U.S. ground forces wouldn't need to assault Baghdad. "The feeling is, they'll be successful in the first phase, and then the next phase won't be necessary, because the regime will fall and a new regime will take over," said a military planner. Indeed, the U.S. intelligence community has predicted that Hussein might even be ousted before a U.S. attack is launched, once it becomes clear in Iraq that such an attack is imminent.

Overall, the plan makes sense by trying both to undercut Hussein's domestic base and to minimize his ability to strike neighbors, said retired Air Force Col. Richard Atchison, an intelligence officer who specialized in targeting during the Gulf War. "In the north, you separate Saddam from his tribal support base; in the south, you hold the area most seditious to the Saddam regime," he said. "Then you can form an Iraqi government-in-waiting with your coalition allies."

Meanwhile, Atchison said, in the west, where there is little except a highway and two Iraqi military airfields and weapons depots, "you protect Jordan and Israel."

This article was discussed extensively in recent days with several senior civilian and military Defense Department officials. At their request, several aspects of the plan are being withheld from publication. Those aspects include the timing of certain military actions, the trigger points for other moves, some of the tactics being contemplated and the units that would execute some of the tactics.

Some of those officials said they see a strategic benefit in disclosing the dual nature of the plan. Discussing its broad outline would help inform the Arab world that the United States is making a determined effort to avoid attacking the Iraqi people, one said. At the same time, he added, it also might help the Iraqi military understand that the U.S. military will be able to destroy any units that resist.

But the entire plan is designed to avoid having to engage in debilitating urban combat in the streets of the capital, where U.S. technological advantages would be degraded and civilian casualties would be inevitable.

In phase one of the operation, the U.S. military would move into the nearly empty western desert bordering Jordan. The purpose of this action would be to keep Israel from being attacked by missiles or unmanned drone aircraft laden with chemical or biological weapons. U.S. troops would look for airstrips and stretches of highway from which drones could be launched. They also would keep a watch for Scud missiles, though U.S. military intelligence analysts consider it unlikely that Iraq has operational Scuds that it could deploy to the west.

At roughly about the same time, the 101st Airborne Division and a similar helicopter-heavy British unit would move from bases in Germany and Turkey into northern Iraq. This is expected to be a largely unopposed movement because northern Iraq is Kurdish and has been largely autonomous since the end of the 1991 Gulf War. The CIA is believed to already be operating there.

Once in northern Iraq, U.S. forces could establish operating bases through which "follow-on" U.S. units could fly in from Turkey to refuel and then launch attacks further south. In particular, this would position U.S. troops for attacks on Tikrit, a city of about 50,000 that lies on the Tigris River, about 100 miles north of Baghdad.

Putting a large U.S. force in northern Iraq would also keep the Turkish military from believing that it has a free hand in dealing with the Kurds, with whom it has been engaged in low-level fighting for years. But this aspect of the operation is mainly expected to be defensive, carried out mainly to keep the Iraqi military from trying to retreat into the north.

In the south, British forces and the U.S. Marines likely would be assigned to seize airstrips and other key facilities in the heavily Shiite section around the port city of Basra, just north of Kuwait. This aspect of the plan "gives the Shiites a chance to get organized," said a former Central Command official. The Shiites adhere to a form of Islam that is different from that of Hussein and most of the people around him, who are Sunni Muslims.

Then, if Hussein is still in power, U.S. tanks would spearhead a multipronged attack on Baghdad and Tikrit, the source of Hussein's strongest support. That part of central Iraq is considered to pose a far more difficult military problem than does the rest of the country, in part because antiaircraft weaponry has been withdrawn from other areas and concentrated there, according to U.S. military intelligence analysts.

The plan resembles the 1989 U.S. invasion of Panama more than it does the 1991 Gulf War, people familiar with it noted. "This is looking more and more like a Panama-style takedown, a Special Operation writ large, but with significant follow-on forces . . . to pacify any bypassed pockets, prevent too many reprisal killings of the Baathists and reduce any holdouts," said Tom Donnelly, a defense analyst at the American Enterprise Institute who is co-author of a history of the Panama operation.

The force being considered to carry out this plan is larger than some senior Pentagon civilians and some air power advocates had advocated earlier this year. People familiar with the plan say that it currently contemplates using about 250,000 Army, Air Force, Marine, Navy and allied personnel. The numbers are drawing fire from some who had argued for a smaller approach that would rely less on conventional Army forces and more on a combination of Special Forces, airstrikes and rebel Iraqis. "This is a classic Army/Tommy Franks plan, and I believe it is their game plan," said one retired Air Force general, referring to Army Gen. Tommy R. Franks, the head of the Central Command, the U.S. military headquarters covering Iraq and the Middle East.

Some military officials said the overall troop size -- which amounts to about half the 500,000 U.S. Army troops deployed for the 1991 Gulf War -- is a powerful signal to the Iraqis that the U.S. government is determined to bring change to Iraq. "They have to be prepared to go in with a large force and must advertise that fact in order to attempt to get Saddam to quit or to get to those who might try to overthrow him," one planner said.

Whatever the reason, Franks is now seen inside the Pentagon as having prevailed in the argument over the size of the force. "I'd say he's a happy camper," said one person familiar with Franks's thinking.

That is a radical shift from earlier this year, when Franks first presented his thoughts about attacking Iraq to Rumsfeld and was ordered to reexamine his assumptions. "He was slam-dunked on his early war plans," said one person familiar with that discussion. "He was told, 'Go off, be more creative, we don't want to put huge forces on the ground, and your time lines are too long.' "

The plan that now exists is a kind of compromise, the planner said, with Franks now offering an approach that is speedier than originally envisioned but with a troop contingent close in size to what he originally proposed. "I think it is a pretty audacious and bold plan," said the Defense Department official who was recently briefed on it.

But even now, the plan isn't set in concrete. Most notably, there are still some concerns among military leaders about which units -- and which nations -- would shoulder the burden of post-victory occupation. "The military piece of defeating the Iraqi army is certainly within our capabilities," Gen. James L. Jones, the Marine commandant, said in a recent interview. But, he added, "there is always the aftermath, and that is one of the great unresolved questions. There are all kinds of questions here." The first one he noted is, "How long are you going to stay?"