Washington ? 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 probably will fall from power before U.S. forces enter the capital, senior U.S. military officials said.

Hedging its bets, the Pentagon also is preparing for the possibility of prolonged fighting in and around Baghdad. Administration war planners expect that, even if the Iraqi leader is deposed from power, there could be messy skirmishes there and in Saddam’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 some 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 would be prepared to go to war if Iraq flouts the new U.N. Security Council-approved resolution.

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 force 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 Saddam. 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 Saddam 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 air strikes 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 under way 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.