The convoy stopped in a single-file line: a half-dozen U.S. armored military vehicles and one gray Nissan pickup truck, all of them idling in a dirt lot in the insurgent-controlled slum called Sadr City.

In the pickup were five members of the Iraqi National Guard, resting up after patrolling with U.S. troops. The men sipped water in the hot midday sun. They wore bulletproof vests but no helmets as they sat in their unarmored truck.

Without warning, an orange fireball engulfed the area, followed by a deafening explosion and then gray smoke that blotted out the sun. When it cleared, the Nissan and the Iraqis inside it were riddled with marble-size ball bearings that had sprayed from a roadside bomb.

"They're dead! All of them are dead!" shouted an American soldier who had rushed to the vehicle.

"Make sure!" shouted another. "See if any of them are moving."

"They're done," said the first, turning away. "They're all done."

Three Americans -- all gunners whose job requires them to stand partially exposed in the rear hatches of the bulletproof Humvees -- sustained wounds, though none that were life-threatening. Dozens of other U.S. troops on the scene escaped unharmed, thanks largely to their vehicles' armor.

The blast, witnessed by a Washington Post reporter riding in an armored Humvee directly behind the Nissan on Monday afternoon, demonstrated the uneven vulnerability of U.S. forces, who are equipped with the most sophisticated weaponry and armor, and their Iraqi allies, who fight the same battles using vastly inferior equipment.

Among the Iraqis, there are frequent complaints that they don't have the tools for the job. About two dozen Iraqi soldiers and would-be recruits have died in the past week in ambushes and bombings. With elections here scheduled for January, the Bush administration has staked its hopes for the country on Iraqis assuming increasing responsibility for security.

Asked if a Nissan pickup afforded sufficient protection in Sadr City, where more than 100 roadside bombs exploded last month, Capt. Haider Yehya, the commander of the Iraqi guardsmen, responded: "No. Those vehicles, those are civilian vehicles. They're not right for the army."

The Iraqis who died were Amar Ali, 22; Walid Younes, 28; Sabah Mujed, 25; and Thamer Ali Majed, 20, according to Yehya. Despite the American soldiers' initial fear that everyone in the truck had died, there was a survivor, the Nissan's driver, Ahmed Khaleb, 25, who sustained severe shrapnel wounds to his face and leg. Khaleb crawled from the truck, soaked in blood, then collapsed in the dirt. He was evacuated to the U.S.-run Combat Support Hospital in Baghdad. There was no information about his condition.

The day began at around 11:30 a.m., when the 2nd platoon of Bravo Company, 2nd Battalion of the 8th Cavalry Regiment, 1st Cavalry Division, made its way up Baghdad's Canal Road to patrol Sadr City, which remains largely under the control of insurgents loyal to Moqtada Sadr, the rebellious Shiite cleric.

The Iraqis were members of the 305th National Guard battalion, which has two companies attached to the 1st Cavalry Division. The company whose men were hit by the explosion, Company C, is nicknamed "The Lions of Freedom."

The vehicles assembled at a traffic median near Sadr City's southern boundary. The convoy then made its way into the slum, steering around huge mounds of festering garbage and past narrow alleys that the soldiers eyed warily as potential hiding places for ambushers. Posters of Sadr were plastered to the sides of buildings.

In this tense environment, children walked along streets, watching the patrol. Some waved at the soldiers and smiled; when the convoy briefly stopped, they surrounded the Iraqi guardsmen and talked with them. Then, as the convoy drove away, the mood changed. Some children threw rocks at the vehicles.

Around 2:45 p.m., the convoy made its way back to a large vacant lot near the southern edge of Sadr City, where it was joined by vehicles from other units patrolling the area. Two M1-A2 Abrams tanks were pointed at the neighborhood.

Lt. Tye Graham, 23, of Pecos, Tex., said commanders had decided to use the lot as a staging area for a raid on insurgents thought to have plotted a bombing in Sadr City that had injured two Americans about two hours earlier.

Behind the Nissan, Sgt. Anthony Stewart, 31, of Sumter, S.C., sat in his Humvee, watching the Iraqi guardsmen. Two were sitting in the rear bed of the pickup; one was swigging water spiked with rehydration powder that the U.S. soldiers had given him. But he was spitting the water into the dirt.

"Look at those guys, they don't know how to drink it," said Stewart. He said later that he thought about getting out of the Humvee and walking over to explain that they needed to swallow the powdered water for it to be effective.

Before he could do that, the air filled with an orange fireball that seemed to erupt about 10 feet to the right of the Nissan.

The smoke from the explosion cleared after about 30 seconds, revealing the carnage.

"We have ING wounded!" Stewart shouted into the radio. "ING are down!"

The truck had offered no protection. The man who had been swigging the water was slumped against the rear of the cab, his eyes open, his body bloodied and motionless. The man next to him also appeared to have been killed instantly; his body lay against the left side of the truck, his right hand spread across his lap. Blood and parts of his brain and skull trickled down the left rear panel.

Inside the cab, two others were dead; a man in the passenger seat had two ball bearings lodged in his forehead.

Khaleb, the driver, managed to open his door and take a few steps toward the company medic, Spec. Justin "Doc" Martin, who was riding in the Humvee in front of the Nissan. But then Khaleb collapsed in the dirt and crawled until the medic reached him.

Martin cut off the man's bloodied clothing and began to treat him for arterial bleeding.

Another soldier shouted that the gunner in Martin's Humvee was also down. The man, who was unconscious, had been blown back into the gunner's hatch.

"We got him woken up," Martin said later. "He didn't know where he was. He didn't know who I was."

Martin turned to another gunner who had been two Humvees behind the Nissan. He cut away the man's shirt, revealing his punctured right shoulder.

"My shoulder, Doc," he said. "I can't feel my shoulder."

U.S. officials requested that the names of the three Americans wounded by the bomb not be released, because their families may not have been notified.

In the meantime, two American soldiers got Khaleb onto a stretcher and placed him in a Humvee.

U.S. commanders quickly began to clear the area, fearing another attack. The truck was abandoned. The Humvees raced out of the southeast side of the lot, a cloud of dust rising in their wake. The two that had led and followed the Nissan headed back down Canal Road in the same order, now minus the pickup.

Within a half mile of the lot, the shrapnel-pierced right rear tire of the front vehicle began to disintegrate. Black smoke and the smell of the burning tire trailed behind it, filling the air. Outside Sadr City, in a more secure area of Baghdad, the two vehicles pulled over to the side of the road. An hour passed as the soldiers struggled to jack up the heavy vehicle and replace the oversize tire.

The skin on the right side of both vehicles was gouged with holes the size of marbles. The right rear windows were also punctured, but shrapnel did not appear to have penetrated either vehicle.

Pfc. Dion Butler, 20, was riding in a rear passenger seat in the Humvee in front of the Nissan. He said he had opened the door slightly before the blast to test it because it had been sticking.

A piece of shrapnel appeared to have entered through the slight opening. It narrowly missed the head of Sgt. Jason Pries, 28, of Rochester, N.Y., who was seated in the front passenger seat. The ball bearing hit the front window, gouging the glass and spreading a web of cracks from the point of impact.

"You almost got me killed, man," Pries joked in relief when Butler said he had left the door open.

Sgt. Nick Varney, of Lancaster, Calif., was driving the Humvee behind the Nissan. He called Sadr City "an IED planet," using the shorthand for the military term improvised explosive device, and said an attack in the lot had been likely because U.S. tanks frequently park there. "It was only a matter of time before the Mahdi militia was going to try to stage an ambush," he said.

The vehicles made it back to Camp Cuervo, a forward operating base about six miles southeast of Sadr City, at 5:30 p.m.

"I've never needed a cigarette more in my life," said one of the soldiers.

"I've never needed to drop acid more in my life," said another.

Lt. Col. Florentino "Lopez" Carter, the task force commander, said that until the Iraqis received better equipment he would no longer send them into Sadr City.

Before the June 28 transfer of political authority to an interim Iraqi government, the Iraqis accompanied the American soldiers on patrols, often taking vacant seats in their Humvees. They now ride in their own vehicles -- not just used pickups but civilian transport trucks and minivans provided by the interim government's Defense Ministry. They use old AK-47s and RPK light machine guns.

"They're still using those old World War II-style helmets," said Carter. "Truth be told, they're better off without them, because they don't provide the ballistic protection that our equipment provides."

Carter said the lack of adequate equipment is "the biggest challenge" to the goal of integrating the Iraqis into U.S.-led operations.

As Carter spoke, Maj. Hugh McGloin, his operations chief, walked into his office. Earlier in the day, McGloin had been wounded by a separate roadside bomb. Shrapnel hit him in the back of his helmeted head. As he bent over, he said, his blast-resistant glasses fell off, then began to pool with blood.

McGloin's head was bandaged. Carter handed him a cell phone to call his family, then examined the injured soldier's helmet.

"That saved your life, bro," said Carter.

Ahmed Khaleb was injured in a blast that killed four fellow Iraqi National Guardsmen as they patrolled Baghdad's Sadr City in an unarmored truck. U.S. forces with them had armored Humvees.Ahmed Khaleb, a guardsman and driver, was hospitalized after the bomb blast. His commander said civilian vehicles are "not right for the army."