For Pfc. Nick McLaughlin, the war began Tuesday night. A rocket-propelled grenade screamed past his amphibious assault vehicle, which plowed headlong into a marshy bank, stopping in its tracks with its back end in the air.

The radio on the wall inside blared to life: "We're taking fire, we're taking fire!" someone shouted. "Small arms and RPGs. If we don't get going soon, we're sitting ducks. Move!"

Up to that point, McLaughlin had not fired a shot in four days in Iraq. But when his platoon commander gave the order to exit the disabled vehicle, the 19-year-old from St. Louis was the first to charge out the back hatch. He took aim at muzzle flashes 200 yards away in the darkness, a nighttime sludge of rain and sand so dense that night-vision goggles were useless.

McLaughlin spent two magazines of ammunition in a 45-minute exchange with a group of attackers lodged behind a four-foot sand berm on the road toward Kut, a city 100 miles southeast of Baghdad. The other 18 Marines on his vehicle spilled out behind him, laying down supporting fire. Mortar men lit up the sky with flares to illuminate the ambushers. But most Marines said they never saw the assailants and had no idea how many there were, where they were from or what became of them.

"Now it's a war," said McLaughlin, a few hours after the firefight, while another amphibious assault vehicle pulled his vehicle out of the muck. "It didn't feel real 'til the first round went by my ear."

Before the ambush, McLaughlin and the rest of the 1st Battalion, 7th Marine Regiment, had spent days after securing a key oil facility in southern Iraq without firing more than a few shots. But they quickly found themselves back at the center of the 1st Marine Division's push toward Baghdad.

To their north, two Marine battalions had stalled in the muddy swampland southeast of the Iraqi capital when their vehicles ran out of fuel. The mission assigned to the 1st Battalion: Escort 55 fuel trucks bound from Kuwait carrying more than 500,000 gallons of diesel through Nasiriyah, where nine Marines were killed in a roadside attack Sunday, a 10th was killed soon afterward and convoys had been hit by snipers.

"Had those fuel trucks not gotten through, the division would have come to a grinding halt," said Lt. Col. Christopher Conlin, the battalion commander. "It was the absolute priority."

When the orders came through, most of the battalion seemed disappointed. Many were expecting to continue pushing north to Baghdad, after a 48-hour period that one Marine called the "most uncomfortable" of his life. It began with a 24-hour road trip up the spine of Iraq and across the Euphrates to the land of the Marsh Arabs. It was the farthest most of the amphibious assault vehicles had ever traveled in a single stretch as the Marines pushed to match advances on the other side of the Euphrates by the Army, which was near Karbala, about 50 miles southwest of Baghdad.

"I am amazed that those things are still holding up," said Gunnery Sgt. Thomas Boore, 31, of Benson, Ariz., as the convoy came to a stop Monday night. "That was the longest day those hogs have ever been through, and they say we have a lot farther to go."

The advance was halted by Tuesday's sandstorm, beginning in the morning with a gusting wind that whipped the talc-like soil of the marshland into a blinding sheet that burned the lungs and reduced visibility to less than 20 feet. The battalion has endured its share of sandstorms at its home base of Twenty-Nine Palms, Calif., but most said this was the worst they had ever seen. The Marines spent 12 hours buttoned up in their hatches. Sand poured in the roof hatch, coating faces with a quarter-inch of fine dust.

They made the most of their predicament, telling dirty jokes and cleaning dust from their rifles with toothbrushes. A voice from the radio speaker, which provided the only connection to life outside the closed amphibious assault vehicle, said, "Is the whole day going to be like this?"

"God, I hope not," another answered.

Twelve hours after the storm started, a light rain began to fall that settled the dust and cleared the air. Early Tuesday evening, the orders came assigning the battalion to protect the desperately awaited fuel trucks that were approaching 20 miles to the south, slowed by the sandstorm.

But while the rain provided relief, it caused another problem: mud. When the battalion reached the convoy, some of the tankers were stuck in mud to their wheelbases and needed a tow. Soon, the mud captured escort vehicles as well. At least 10 Humvees got stuck.

The first shots came just minutes after the tankers started to move. Most of the firing whizzed overhead, with some bullets pinging off the side of the lightly-armored vehicles. The mood inside was tense: The vehicles' armor can stop an AK-47 assault rifle's 7.62mm round, the Marines knew, but an RPG landing right could tear it apart like it was a tin can.

Marines poked their heads from the vehicle's half-opened top hatches and returned fire. From the gun turret of amphibious assault vehicle B212, Boore squeezed off a few rounds from a .50-caliber machine gun. He tried the Mark-19 grenade launcher, but it jammed, its mechanism filled with sand.

The fighting went on sporadically until dawn, but no Marines were injured. All the fuel tankers, except one that broke down, made it north to the units that needed them. "We were lucky," Conlin said.

Small units surveying the fields this morning where the first shots were fired found virtually no trace of an Iraqi troop presence. Lance Cpl. Bradley Rose said that just after he fired a round, he saw a man with an AK-47 fall on the horizon. But when the battalion moved on to its next destination, the Marines still did not know the identity of their attackers.

Said Cpl. Geoff Burns, 21, of Tucson: "We were shooting at the shadows."

A U.S. Marine shouts orders to colleagues after their convoy came under attack in southern Iraq, resulting in a firefight with unknown assailants. U.S. Marines in armored vehicles escort tankers through the desert north of the Iraqi town of Nasiriyah.