As dawn broke over the arid landscape and three days of sandstorms gave way to a crisp clear morning, the sound of explosions broke the stillness. Soon, plumes of black smoke were rising to the east and north.

It had been a calm night on the perimeter of this U.S. Army encampment southwest of Baghdad, but now Iraqi armored vehicles were probing the lines. It was an attempt, officers of the 3rd Infantry Division figured, to discern where the U.S. force was so that gunners could try to shell the tanks, armored personnel carriers and support vehicles spread out across the desert.

Spotted by M2 Bradley Fighting Vehicles stationed along the perimeter, the two Soviet-era T-55 tanks and two armored personnel carriers came under attack from Air Force A-10 Thunderbolt II tank-killing planes and were quickly destroyed, U.S. officers said.

"They were trying to do reconnaissance on our positions so that they could call in artillery on us," said Maj. Roger Shuck, the operations officer of the 3rd Battalion, 15th Regiment.

For now, the army division spearheading the U.S. invasion of Iraq has stopped its drive toward Baghdad, taking time to bring up its supplies, repair its vehicles and rebuild its combat power before resuming its campaign. That makes the units more vulnerable, and commanders are warning the troops to be especially vigilant.

"Several enemy vehicles tried to probe our lines," Lt. Col. Stephen Twitty, the 3rd Battalion commander, told his troops today. "Make sure we do not become complacent as we sit in a defensive posture."

He ordered the troops to dig defensive positions and to move about the assembly area with at least two vehicles at a time. He stressed the need for "defensive security" to avoid casualties. As of this afternoon, only one soldier in the division had died in combat, the victim of a sniper as a column was passing southwest of Najaf this week, Shuck said.

But even while key combat units of the division have been holding ground, they have continued to engage Iraqi forces, mainly militias loyal to President Saddam Hussein, and to call in airstrikes against them. In one of the latest actions, U.S. Air Force planes bombed the Najaf headquarters of Hussein's ruling Baath Party, and tanks shelled survivors who fled the airstrike. That attack and those on other targets in the city resulted in 200 Iraqis killed, according to a military intelligence report issued tonight.

The intelligence report said the Iraqis appeared determined to take back at least one of the four bridges across the Euphrates River in the Najaf area that the 3rd Infantry Division now controls.

Officers said that appeared to be the objective of a large convoy -- consisting of civilian pickup trucks and sport-utility vehicles -- that was reported bombed by a B-52 Wednesday after it headed south out of Baghdad and made its way toward one of the bridges. In addition to militiamen, the convoy contained members of the Special Republican Guard, the elite unit responsible for protecting Hussein, the latest intelligence report said.

The report said a bomb damage assessment was not completed.

Officers said they were surprised by the tenacity of the militiamen, the main opponents of U.S. forces so far, but stressed that they have not posed much of a challenge to date.

"I really don't know what their thinking is, because they're not using any kind of tactics at all," Shuck said. For the most part, he said, they just drive their pickups straight at M1 Abrams tanks and Bradley Fighting Vehicles in "direct frontal assaults" aimed at getting close enough to fire a rocket-propelled grenade. They rarely succeed, usually getting picked off well before they come into range.

Shuck called the long convoy, with as many as 1,000 vehicles, practically "suicidal," given U.S. air power. "There's no way you have 1,000 vehicles in a column. They must have thought that because of the bad weather and high winds we wouldn't be able to see them. So they figured they could reposition."

But the Air Force's JSTARS airborne radar system quickly spotted the convoy, despite the sandstorm, and airstrikes were called in.

After the foiled attempt to probe the U.S. lines and occasional barrages of 3rd Division artillery fire during the morning, Tactical Assembly Area China 4 -- the name given the area by the 3rd Battalion troops -- remained calm the rest of the day.

For the first time since the U.S. force arrived, the arid plain was clear of flying sand. From the Bradley of Capt. Ronny Johnson, 37, of Dallas, the other groupings of army vehicles looked like miniature cities in the distance. Despite the relative quiet, before day's end Johnson's Bravo Company picked up and moved again, trying to stay one step ahead of any hostile forces that might be out there.

Trouble was, the order to move came after the men had already dug their foxholes. "They aren't happy campers right now," Johnson said.

Capt. Ronny Johnson, a company commander, makes a radio call in his Bradley Fighting Vehicle while guarding an encampment at daybreak in central Iraq.