Moving along a road lined with sand berms and waving Iraqi men, U.S. Marines sent thousands of troops across the Tigris River today and pushed northward to within striking distance of Baghdad.

A vast column of tanks, Amtrac amphibious assault vehicles and Humvees churned more than 70 miles through Iraq's central desert in an advance that met little opposition. Along the way, they seized an airfield that could be useful for attacking the Iraqi capital and reached a bridge that allowed Marines and their armored equipment to cross the Tigris -- the last major geographic obstacle before the flat, spreading suburbs south of Baghdad.

Once on the other side of the river, lead elements veered to the northwest and joined another column of Marines in an advance that moved to within 60 miles of the capital, according to reports from the scene.

"This was the big push," said Lt. Col. Christopher Conlin, commander of the 1st Battalion, 7th Marine Regiment. "The biggest day will come when we reach the outskirts of Baghdad, but if you're Saddam, now we're in your backyard."

Fears of chemical attack rose as the U.S. forces moved nearer to Baghdad. Marines, already in suits designed to protect against chemical or biological weapons, were ordered to sleep tonight with protective boots on as well -- the first time the precaution was obligatory since the invasion began March 20 -- and to keep their protective gloves in their right-hand trouser pockets and their gas masks attached to their hips and ready to pull on.

Conlin's battalion seized the airfield, fewer than 10 miles south of the Tigris, which he said Marines or other U.S. forces could use to re-supply front-line units or establish an air base to attack Iraqi units farther north. When the Marines reached the airfield around 4 p.m., it was largely abandoned. The fleeing Iraqi army had left the two paved runways littered with destroyed armored personnel carriers and other military vehicles.

"They were trying to draw us into bombing the runway so we couldn't use it," Conlin said, standing on the tarmac while Marines scrambled out the back of their Amtracs to perform a cursory inspection of the vehicles. "Once we clear the runway, it will open up another staging ground for us."

The airport was secured in a matter of minutes, as a dozen Amtracs rolled onto the runway and unloaded Marines to provide security. Combat engineers and explosive-ordnance disposal teams, EODs in Marine jargon, were called in to inspect the vehicles for booby traps and clear away the abandoned vehicles.

Small bombs that resembled green Coke bottles with a white streamer attached were found in the area of the airport, most likely dispersed by U.S. artillery batteries or aerial bombardment blasting the area before the ground attack, Conlin said.

Units from the 5th Marine Regiment were the first to cross the bridge, while the 3rd Battalion, 7th Marine Regiment held the road at both ends to prevent any attack.

As they made their way north, the Marines encountered only light resistance. They began the day in the Fawar region, east of the city of Najaf, and drove along the paved main highway. The bleak desert landscape slowly gave way to more fertile-looking fields and stands of palm trees.

For the first time since they left Safwan, on the Kuwait border, the Marines saw towns large enough to boast two-story buildings. Most were sprayed with the notation "EOD cleared" and a date from the last few days, indicating the explosives disposal teams had been through. A large mural of President Saddam Hussein was disfigured with the same mention, "EOD cleared," and a large "R.I.P." sprayed on at the bottom.

Iraqi men standing by the side of the road in their long robes and red-and-white-checkered headdresses waved greetings as the convoy passed. A few women, clad in long black robes, stooped to gather unidentified objects from the ground; they did not look up to wave.

On one stretch of highway, combat engineers found minefields and marked them with white electrical tape. Rather than clear the mines, they devised routes to drive around them. Combat engineers also laid a floating bridge that the Marines used to cross over a canal -- called, perhaps inevitably, the Saddam Canal -- on approaches to the Tigris.

Some obstacles appeared to have already been destroyed by the time most of the units reached their objectives. At one spot along the highway, six dead bodies in Iraqi military uniforms were lined up in front of a burned-out pickup truck. Piled next to them was a stack of rocket-propelled grenades and assorted other small arms.

The Marines seemed surprised that they did not encounter more resistance along the way. "It makes you wonder where the Iraqi army is," said Lance Cpl. James Boas, 19, an Amtrac crewman. "I expected we'd have to fight somebody."

The long drive north marked a final stage in what has been one of the longest overland invasions ever conducted by the Marine Corps, beginning at the Kuwaiti border about 250 miles to the south. "The vehicles are still holding up okay," said Gunnery Sgt. Thomas Boore. "But the big fight definitely hasn't happened yet."

U.S. Marines with the 1st Division secure an Iraqi army outpost near a town on the Tigris River. Thousands of Marines crossed the Tigris, securing towns and an airfield on the way.