The warning came over military radios as soldiers of the Army's 3rd Infantry Division were cleaning their weapons, servicing their vehicles and bringing in food and water. The Air Force was about to bomb suspected Iraqi chemical weapons trucks. Put on your gas masks, the soldiers were ordered, get in your vehicles and button up.

They did, scrambling into Humvees, trucks and armored vehicles and closing the doors. Shortly afterward, the all-clear was given. But an hour and a half later came another radioed warning: "Gas! Gas! Gas!" The soldiers dutifully took cover again.

This is life in the desert for men and women of the 3rd Infantry Division as they pause in their advance on Baghdad to regroup and try to rest.

The Air Force struck three tanker trucks in the nearby desert, setting them on fire. Commanders said it was not immediately clear what the tankers contained, but said that the trucks were moving dangerously close to U.S. lines in the desert southwest of the Iraqi capital.

They said the all-clear was sounded despite the uncertainty about the trucks' contents because the wind was blowing smoke from the burning tankers away from them.

"We felt there were potential WMD [weapons of mass destruction] products out there," said Capt. Felix Almaguer, 29, of Summit, N.J., intelligence officer for the division's 3rd Battalion, 15th Regiment. "Anything with a tanker associated with it moving out there on its own" is suspicious, given that the Americans are camped out in the middle of nowhere, he said.

All afternoon, black smoke billowed into the clear sky from the trucks, possibly indicating that they contained fuel.

But the possibility of chemical attack continued to preoccupy U.S. commanders, especially after a patrol found an Iraqi observation post in the zone of one of the 3rd Infantry Division's tank-heavy battalions. Two Soviet-style gas masks and chemical suits were discovered in the post, along with a sophisticated French-made radio, officers said.

Some soldiers in the division, which has suffered few casualties, are griping over the extended stop in the desert. But commanders say the unit is making good progress.

"Although we have slowed down our movement, the war continues to be going well," Lt. Col. Stephen Twitty, commander of the 3rd Battalion, told his company leaders today. But he warned, "Every day we sit here, we continue to get vulnerable. The potential for an artillery strike is pretty high."

So he ordered stepped-up "counter-reconnaissance patrols" every hour in the evening to try to intercept any attempts to probe the U.S. perimeter. The Iraqis are going to "probe our lines to try to get the first read" on which units will lead an upcoming attack and where they will strike, he said.

And he issued another warning: vipers and scorpions have been found in some of the battalion's positions. "The vipers are extremely poisonous," he said. "Tell your soldiers to leave the critters alone."

Despite having to drop its original plan to race north straight to Baghdad and trigger the fall of President Saddam Hussein, the division remains in a strong position, the intelligence officer said, and is adapting to new conditions.

"We're not trying to put a square peg in a round hole," said Almaguer. "We're not blindly following a plan. If anything, in the Army we're flexible. There's nothing to indicate anything but the fact we're having tremendous success. I see us overwhelmingly hammering [Hussein] and him having almost no effects on us."

The basic plan hatched while the units waited in Kuwait was to race north so rapidly that the Hussein government would be panicked into collapse.

The 3rd Division's 2nd Brigade, commanded by Col. David Perkins, sped north in two main groups. One, called "Heavy Metal" and consisting mainly of M1-A1 Abrams tanks and M2-A2 Bradley Fighting Vehicles, barreled across the desert from Kuwait at top speed.

The other group, code-named "Rock 'n' Roll" and including the wheeled vehicles, supply trucks and slower tracked vehicles, swung out to the west and linked up to little-used roads that took it north. The lead elements of the columns met up near Samawah and headed further north.

"We outran our logistics supplies, and now we're waiting for them to catch up," said Capt. Ronny Johnson, commander of Bravo Company of the 3rd Battalion. Meanwhile, the units will take the opportunity to build their combat power back to full strength and give airstrikes a chance to further pummel Republican Guard troops protecting the southern approach to Baghdad, he said.

According to Almaguer, airstrikes already have reduced key units of the Republican Guard's elite Medina Division by a third or more. "They're taking a well-deserved beating right now," he said.

A pilot with the Michigan Air National Guard drops flares from his A-10 Thunderbolt aircraft as decoys for divert surface-to-air missile attacks after a strafing run against Iraqi forces.U.S. Army 3rd Infantry Division soldiers put on their gas masks after U.S. forces attacked an Iraqi tanker truck containing unknown fluids near Karbala.Residents of Al Kifl, a town on the east bank of the Euphrates, listen to an interpreter from the 3rd Infantry Division explain that they will soon have freedom but for now their safety requires that they avoid U.S. soldiers and tanks.