Just seven months ago, Capt. Glenn Kozelka and his men from the Army's 10th Mountain Division fought al Qaeda terrorists in the mountains of Afghanistan. But last week, as he commanded a furious mock assault on the U.S. military's most sophisticated urban training ground, he began to understand why Army doctrine describes city fighting as "primordial combat."

He had lost an entire squad to mortar fire, a sniper atop an adjacent building was picking off his soldiers in the street one by one, and a rocket-propelled grenade had just slammed into the next room, killing or wounding everyone inside.

"We call it three-dimensional warfare," Kozelka said early one morning. "You can be shot from all around."

War planners at the Pentagon understand this geometry only too well: They foresee a battle for Baghdad, a sprawling city of 5 million people, as one of the most difficult and unsettling aspects of any invasion of Iraq. The last thing they want is to mount a full, frontal assault on the city because of the likelihood of high casualties, both military and civilian, and the demands it would make on already strained manpower.

Lt. Gen. Edwin P. Smith, in last month's issue of Army magazine, called urban warfare "the great equalizer." The U.S. military is trying to minimize that equalizing effect, both in its planning and its training.

Defense Secretary Donald H. Rumsfeld on Oct. 1 directed Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, to conduct "a top down national review and theater review of assets involving urban warfare." Two weeks ago, retired Marine Col. Gary Anderson, a Pentagon contractor, briefed Rumsfeld's aides on the results of a two-year urban warfare analysis and recommended that 36 infantry battalions -- about 18,000 troops, or roughly half the Army's infantry force -- receive intensive training in urban operations right up until the time they deploy to the Persian Gulf.

All Army and Marine infantry units have routinely been given training in urban warfare, but recently that training, such as the exercise Kozelka and his troops were on, has increased in intensity and focus, with an eye toward a conflict in Iraq.

The Big Unknown

Recent experimentation by the Marine Corps has shown that battlefield casualties exceed 30 percent in simulated urban operations involving troops who receive, on average, only about two weeks of urban combat training per year, said retired Marine Col. Randy Gangle, an official at the Marine Corps Warfighting Laboratory.

Senior Iraqi officials have already said they would try to lure U.S. forces into Baghdad, acknowledging that the Persian Gulf War in 1991 taught them the folly of fighting in the desert against superior American armor and air power. Bluffing or not, the Iraqis understand that the U.S. military's overwhelming technological advantages are to some extent nullified in cities, where buildings shelter enemy forces from reconnaissance aircraft and satellites and the presence of civilians makes the use of even the smartest bombs infinitely more difficult.

The big unknown confronting senior defense officials is whether the Iraqi military would fight to save President Saddam Hussein -- and, if it did, whether it would have the discipline and leadership to fall back into the Iraqi capital and extract a heavy price from the U.S. invaders, as Chechen rebels did when Russian forces invaded Grozny in 1994.

Military analysts inside and outside the Pentagon do not think that Iraq's military can or will put up much of a fight, but even a limited number of engagements, most likely against Hussein's Special Republican Guard, could be nasty affairs.

"It is very unlikely that we could become involved in any type of urban warfare and not see young American men and women fight and die," said Anthony H. Cordesman, a former defense official now at the Center for Strategic and International Studies. "The worst case in urban warfare is a bad case indeed."

The Army's urban warfare training manual quotes an Israeli officer in starker terms: "Every room is a new battle. . . . Avoid cities if you can. If you can't, avoid enemy areas. If you can't do that, avoid entering buildings."

In addition to posing the risk of casualties, urban operations also require extremely large numbers of troops. One recent Marine scenario that used Chicago as a battle template determined that it would take the entire Marine Corps to clear and hold the city. Far from that kind of block-to-block engagement, Pentagon strategists envision cordoning off Baghdad, providing escape routes for civilians and surrendering military personnel, and striking critical facilities whose loss, over time, should make the city, and Hussein's government, fall.

Navy Capt. Tom Johnston, head of the Center for Joint Urban Operations at the U.S. Joint Forces Command in Norfolk, said the U.S. military has progressed since World War II from laying siege to cities to waging "effects-based operations" that seek to destroy an enemy's will without harming large numbers of civilians or devastating infrastructure.

"Are we prepared to fight in the urban environment? Sure, but it's going to cost us," said Johnston, explaining that the new organization he commands is working to ensure that any necessary ground attacks "cost less and less and less."

But any engagement in and around a city the size of Baghdad would almost inevitably be characterized to some extent by poor communications, elusive targets and enemy forces typically no more than 50 yards away, Army officers believe. It is for these unforeseeable difficulties that the troops train at Fort Polk.

Laser Tag and Grim Lessons

Kozelka, a 29-year-old from LaCrosse, Wis., said his men are ready for combat in Iraq after a year in which they went from their home base at Fort Drum, N.Y., to Uzbekistan and then to Afghanistan for a sweep of the eastern mountains there in March, the last major ground engagement of the Afghan war and the first involving large numbers of U.S. forces.

He and his men arrived at Fort Polk's Joint Readiness Training Center in early October with two battalions -- about 1,000 troops -- from the 10th Mountain Division's 2nd Brigade for a regular three-week training rotation that began with live-fire exercises and ended last week with a 10-day force-on-force war.

The simulated combat, which costs more than $1 million a day to wage, involves what is probably the world's most sophisticated game of laser tag against an opposing force, fought over a battlefield in central Louisiana, 15 miles long and 10 miles wide, part of which consists of 28 buildings arrayed across the equivalent of three city blocks.

The Army takes great pains to simulate the strain of actual combat. Every soldier wears a laser sensor that beeps when he or she is shot. Once the sensor sounds, a soldier opens an envelope containing a card that describes how badly he or she is hurt.

Medical personnel must evacuate wounded soldiers from the battlefield in time to treat their wounds. Soldiers who die are taken to a holding area, where they are made to do manual labor to underscore the point that dying is never fun.

The 2nd Brigade's combat training began with five days of operations against an insurgent force like al Qaeda, switched to a defensive operation against a more conventional invading force with tanks and other armored fighting vehicles, and ended with a night assault on the urban battleground.

A battle plan developed by brigade commander Col. Kevin Wilkerson, 43, who has fought in Grenada and Afghanistan, called for Kozelka's company to execute the all-important breach of the perimeter defenses. This would come after Kozelka's company and five others had traveled nearly 10 miles by truck through enemy country and then marched through two miles of swamp and heavy woods.

Almost nothing went as planned. The enemy compromised the battalion's radio network, and an enemy armored vehicle machine-gunned the convoy and killed two squads of engineers who were supposed to help Kozelka's men cut through the concertina wire around their assault point, Building 13, which they had planned to storm and use as a company command post.

By the time Kozelka made it inside, after 2 a.m., his forces had fallen prey to all the hazards of city combat -- an unseen enemy, fire from guns high in buildings, and maximum confusion. "Hey, captain, I've got six personnel in first platoon left alive," Spec. Matt Floyd, a radio operator, yelled at Kozelka after the rocket-propelled grenade attack on their position.

"Okay, keep 'em alive," Kozelka yelled back.

Commanding from a room littered with casualties, Kozelka was busy calling in suppressive fire on the next building over and warning soldiers over the radio of a machine gun on the roof of another building across the street.

"It's lighting us up," he said.

After a few hours of continuous combat, Kozelka and the remnants of his company captured the post office building across the street. During a lull, Kozelka relaxed and started chatting with an observer. Then, an Opfor soldier burst through the door and shot him. A half hour later, commanders called a halt to the exercise.

"This battlefield throws everything at you all at once," Wilkerson said, mingling with Kozelka and other soldiers as the smoke cleared. "Now, you've done another scrimmage, you've learned how to fight this war, and you'll do it better when there's live bullets."

Army soldiers, seen through night-vision gear, undergo urban warfare training at the Joint Readiness Training Center in Fort Polk, La. The Army calls city fighting "primordial combat."