The U.S. military is assembling a ground force for a possible invasion of Iraq that could exceed 100,000 troops and include three to four heavy Army divisions, an airborne division, a Marine division and an assortment of Special Operations forces, according to defense officials and analysts.

Although the exact makeup of the ground force has yet to be decided, the Army has summoned commanders of four of its best-equipped and most capable divisions for an exercise called Victory Scrimmage at the end of the month. In it, commanders will use computer simulations to run through Iraq war scenarios, defense officials said. The units include the 1st Armored; the mechanized 1st Infantry; the 1st Cavalry, with its mix of tanks and helicopters; and the helicopter-heavy 101st Airborne.

The 3rd Infantry Division received orders last week to deploy to the Persian Gulf, as did elements of the 1st Marine Expeditionary Force. The 3rd Infantry, based at Fort Stewart, Ga., already has one of three brigades in Kuwait, and the Marine force, based at Camp Pendleton, Calif., has 1,000 Marines from its headquarters staff in Kuwait.

The military buildup in the Persian Gulf has been underway for some time, but has accelerated as the Jan. 27 deadline for the first major report by U.N. weapons inspectors to the U.N. Security Council approaches.

Although it has generally been understood that the size of the U.S. force could reach as high as 250,000, the approximate mix of combat units that would be involved in an invasion of Iraq has started to come into focus only since Defense Secretary Donald H. Rumsfeld signed a major deployment order on Dec. 24. Additional deployment orders are expected this week, according to defense officials.

Numerically, a U.S. invasion force of about 100,000 soldiers would be roughly equivalent in size to Iraq's Republican Guard, with about 80,000 troops, and Iraqi President Saddam Hussein's elite Special Republican Guard, with about 15,000 troops. There are an additional 300,000 soldiers in Iraq's regular army.

But the U.S. ground force would have enormous advantages in technology, firepower and mobility, and would almost certainly begin any invasion with the benefit of near total air supremacy, according to defense officials and analysts.

The 1st Armored Division, based in Wiesbaden, Germany, has 159 M1A1 Abrams tanks, 173 Bradley Fighting Vehicles, 36 Paladin howitzers, 18 Multiple-Launcher Rocket Systems and 18 Apache helicopters. The M1A1 has almost twice the range of Iraq's remaining 600 T-72 tanks, which can be targeted and destroyed by laser-guided Hellfire missiles from miles away by the Apache gunships.

The 101st Airborne Division, based at Fort Campbell, Ky., has more than 70 Apaches and more than 100 Blackhawk and 40 Chinook troop transport helicopters, enabling it to stage 100-mile air assault missions involving 4,000 soldiers in a single operation.

"If we get three heavy divisions, plus the 101st and a Marine division, they will manhandle the Iraqi military in 21 days," said retired Army Gen. Barry McCaffrey, who commanded the 24th Infantry Division during the 1991 Persian Gulf War.

But McCaffrey said he was concerned that some civilian leaders in the Pentagon might try to invade Iraq with as small a force as possible, keeping heavier forces in the rear as reinforcements.

"The forces we've apparently got targeted on Iraq are more than enough to do the job, but the opening moves shouldn't be modulated force to see if we can do it on the cheap," McCaffrey said. "The biggest mistake you can make in boxing and combat is to lead tentatively into battle, instead of dominating it from the start."

One senior defense official said there is no "light" invasion option still on the table. "The goal of armed conflict is not to defeat your enemy," the official said. "It's to take away your enemy's will to fight. And the best way to take away your enemy's will to fight is to bring overwhelming force to bear."

Another senior official said that there is a "debate about risk at every level of planning." But the official added: "They all know that mistakes in planning yield dead bodies on the battlefield."

Retired Army Maj. Gen. Robert H. Scales Jr., a former artillery commander in the 82nd Airborne who also headed the Army War College, said he hopes -- but isn't convinced -- that the invasion force being contemplated is large enough that Iraqi commanders will believe the United States has enough combat power to overthrow Hussein.

"You've almost got to put yourself in the shoes of an Iraqi general who says, 'I have two alternatives here: remain loyal to Saddam Hussein, for fear that he will shoot my family, or trust once the Americans cross the border that they have the will not only to fight against me, but carry the campaign to Baghdad and Tikrit and take care of my boss,' " Scales said. Tikrit, a city northwest of Baghdad, is the ancestral home of Hussein and the base of his power.

A key assumption underlying the U.S. war plan is that Hussein's government would be rocked from the outset by a massive air bombing campaign, coupled with simultaneous Special Operations attacks against airfields and sites of weapons of mass destruction.

The air war, which could involve as many as 500 to 1,000 sorties in the first day, will strike leadership targets, presidential palaces, air defenses, weapons facilities and concentrations of Republican Guard and Special Republican Guard forces across the country, as well as regular military units that resist, defense officials and analysts said.

The air war would be designed to spare traditional infrastructure -- bridges, rail lines and industrial plants -- as well as population and cultural centers, they said.

The Air Force and Navy have more than 1,000 aircraft capable of dropping laser- and satellite-guided bombs and attacking multiple targets per mission. At the beginning of the 1991 war, by contrast, only 100 Air Force planes -- and almost no Navy aircraft -- could drop precision-guided bombs.

The development of the satellite-guided bombs since 1991 gives U.S. aircraft the ability to strike targets with precision in all weather conditions, day and night. They have also enabled lumbering B-52 bombers produced in the 1950s and B-1 bombers built at the height of the Cold War to drop satellite-guided bombs from high altitude in a close air support role for ground troops.

"I am very confident that with rapid dominance, we will take down Saddam very quickly," said retired Air Force Lt. Gen. Thomas G. McInerney. "Clearly, the air war -- since we are 10 times more effective in the air than we were in Desert Storm 1 -- will be a shorter war, particularly with the simultaneity of ground forces."

But Robert A. Pape, a professor at the University of Chicago who is a noted air power skeptic, said that the Air Force strategy for "decapitating" a government by bombing strategic targets has been tried four times over the past 16 years, "and it has failed or backfired each time."

Pape referred to the 1986 bombing of Libya, the 1991 Gulf War, the 1998 airstrikes against Iraq and the 1999 NATO war against Yugoslavia over Kosovo.

"Today, decapitation is even less likely to work," Pape said. "Saddam knows we are coming, and has undoubtedly planned exactly for this scenario. Perhaps we could get a lucky shot, but air power alone is hardly a plan for certain victory."

Soldiers from Fort Bliss, Tex., load onto a railcar vehicles that carry Patriot PAC-3 missile launchers. More than 100,000 troops could be part of a ground force for an invasion of Iraq.