At the gateway to Baghdad, U.S. Marines began an urban battle today against an adversary they had trouble finding.

Slowly, methodically, seeking to draw out Iraqi forces that have been targeting them in guerrilla-style raids, the Marines explored the suburban sprawl between the Iraqi capital and Salman Pak, a small town whose airfield the Bush administration has described as a training site for Iraqi commando squads. The 1st Battalion, 7th Marine Regiment, searched buildings, blew up vehicles and patrolled the streets. But the Iraqi fighters who U.S. intelligence said were lurking in the area never showed their hands.

The frustration was a taste of what may lie ahead as the slow-step tactics employed here in Salman Pak, 15 miles southeast of downtown Baghdad, are extended to the capital, with its 5 million inhabitants, thousands of militia and military defenders and President Saddam Hussein's government.

"We're clearing out the region, zone by zone, trying to track these guys down," said Maj. Dan Healy, the commander of Baker Company, which led the battalion's sweep through the leafy neighborhoods this afternoon. "But we got indications from some of the civilians we spoke with that they leave town for the day and come back to attack at night. Clearly today, they didn't want to come out and fight."

After a two-week slog across the sparse deserts of southern Iraq, the Marines arrived Friday night in a densely populated region where months of training had told them to expect high casualties and fierce fighting. Commanders briefed their troops that they would be confronting a volatile mix of guerrilla and regular military forces. For the first time since the war began, the Marines unpacked thermobaric rounds, used to pound through stone caves in Afghanistan, that they said can level a building with a single shot.

"It will be hard fighting," Healey, 36, of Worcester, Mass., told his troops, before an operation that began at the height of the midday sun. "The enemy now has nothing to lose."

But the Marines encountered virtually no resistance as a long column of armored vehicles clanked through a 20-square-mile region in a single afternoon. In one of the few exceptions, Marines south of Baghdad struggled to subdue a force of foreign fighters, mostly Jordanians, Egyptians and Sudanese, the Associated Press quoted a Marine commander as saying.

After two weeks of war in which the Marines have encountered only sporadic fighting, many said they still do not know what to expect from an enemy they have rarely faced in a conventional battle.

"Every time the ramp drops, we're not sure what resistance we'll see," said Lance Cpl. Daniel Vigil, 20, of Modesto, Calif. "We get briefed about the operation, but things always seem to change."

The operation began with an hour-long barrage of 155mm howitzer shells that traveled directly over the heads of the advancing Marines, shaking the ground as the rounds impacted several miles away on the horizon.

As the Marines waited for the order to move into the suburbs, a steady stream of civilian vehicles headed away from the Iraqi capital, their trunks packed with personal belongings. Marines conducting searches determined that many of the people were fleeing in anticipation of the fighting to come.

The convoy moved west along a major highway leading toward Baghdad, before snaking off through a patchwork of canals and ponds. Marines riding in the passenger seats of Humvees kept the door open and their machine guns pointing out at the tree line. At 3 p.m., they passed under an archway marking the beginning of the zone they were assigned to patrol. At the base of the white stucco structure was a mural of Hussein wearing a checkered head scarf and another of him pointing a rifle into the air.

The convoy proceeded down the village's main street, stopping in front of an official-looking white stone building on a street lined with palm trees that formed a shady canopy and welcome protection from the pounding sun. Squads of Marines poured out of the rear hatches of their Amtrac amphibious assault vehicles and began foot patrols in and around buildings that appeared to be abandoned. A few stood guard in front of a large white mosque with blue stained-glass windows.

First Sgt. Dave Jobe led three rifle-toting Marines to patrol the building's perimeter, stopping on the way back to his vehicle to drop a hand grenade into a military-style jeep, engulfing it in flames. Another Marine shinnied up a steel flagpole to snatch an Iraqi flag as a souvenir.

Sandbags lined the roofs of many nearby buildings and vehicle-size fighting positions had been dug along several side streets. A basketball court was pockmarked by craters that seemed to have been caused by impacting munitions.

Everywhere the Marines went they encountered civilians waving white flags to signify that they were not a threat, creating a few tense standoffs, as Marines have been warned in recent days about Iraqi soldiers posing as civilians.

Tanks drove at the front and rear of the convoy, their 155mm cannons scanning the horizon for targets, while Humvees equipped with .50-caliber machine guns or TOW missiles cruised around. Three-man mortar teams raced from their Amtracs and dug hasty holes for their mortar tubes, before packing up their equipment when no targets could be found.

To try to provoke a response from an enemy that they believed to be lying in wait, they fired dozens of rounds at suspected enemy targets such as a local Baath Party headquarters and an army barracks, hoping that someone would fire back. No one did.

U.S. Marines riding in a convoy of Amtracs and other armored vehicles pass the bodies of Iraqi soldiers as they head north on a road toward the capital. Marines escort suspected Iraqi deserters on the main road to Baghdad. U.S. Marines from Lima Company of the 7th Marine Regiment take a break at the end of a two-week trek across the Iraqi desert before pushing north toward Baghdad.U.S. Marines guard an intersection on the outskirts of Baghdad, where the body of a man killed the previous night lies in an abandoned vehicle. American forces are entering suburban areas of the capital slowly, seeking to draw out the remaining Iraqi forces. Commanders warned their troops to be ready to confront a volatile mix of guerrilla and regular military forces.