"Our guys are able to see the skyline. That's how close we've gotten," the official said, speaking on condition of anonymity. 

But confidence over the steady northward advance was tempered by fear of the unknown: What does Saddam Hussein's regime have in store for coalition forces as they approach the capital? 

Lead U.S. infantry units donned their chemical suits after capturing a bridge 40 miles southwest of Baghdad. Some Marines began adding their protective boots to the suits they already wear, and Marine helicopter pilots were advised for the first time to be ready to don chemical suits at a moment's notice - now that the so-called red zone, the range of guns and missiles defending Baghdad, has been breached. 

U.S. officials warned that a cornered Saddam might resort to unleashing his worst weapons. "There may be a trigger line where the regime deems (a) sufficient threat to use weapons of mass destruction," said U.S. Brig. Gen. Vincent Brooks.  An officer with U.S. Central Command explained that the "red zone" begins on an imaginary line running east from Karbala, about 50 miles south of Baghdad on the Euphrates River, to Kut on the Tigris River southeast of Baghdad.  The United States believes Iraq has mortar shells, artillery and short-range missiles capable of carrying chemical weapons, including the FROG-7 - used to carry mustard gas during the Iran-Iraq war - which has a 40-mile range.

A U.S. soldier from the 1st Brigade of the 101st  Airborne Division stands on the ready in a convoy as smokes billows from a truck loaded with explosive was destroyed by  helicopters in Najaf, Iraq, Wednesday, April 2, 2003. The 101st cleared  downtown Najaf Wednesday.  (AP Photo/Jean-Marc Bouju)

One thing was certain to be waiting for coalition forces in Baghdad: whatever remains of the Republican Guard, Saddam's best-trained and best-equipped forces. U.S. officials said Wednesday that two of the six primary units - the Medina armored division and the Baghdad infantry division - had been largely eliminated as an effective fighting force. 

Four others are still somewhat intact, with two estimated at 70 percent effectiveness and two somewhat less, according to a defense official, who spoke on condition of anonymity. 

A spokesman for the Baghdad infantry division claimed that only 17 men had been killed and 35 injured since fighting began. He said the division was in excellent fighting shape and would teach the enemy "lessons in the art of fighting." 

One sign of Iraqi disarray came in Numaniyah, 40 miles southeast of Baghdad, where Marines who took a key bridge over the Tigris River met little resistance and found the side of the road littered with abandoned Iraqi military uniforms. 

"It looks like a lot of guys threw off their boots and threw off their uniforms and got the hell out," said Lt. Michael Belcher of the 5th Marine Regiment. One man tore off his Iraqi army uniform and donned a brown robe. They quickly seized him. 

There were other signs that Iraqi forces had fled in a hurry. Soldiers found mortar ammunition, abandoned rifles, an unused artillery piece in a field. 

Beside a small marsh, Marines with bayonets fixed on their M-16 rifles stood over a group of 40 Iraqi men sitting on the ground. Four of the Iraqis had their hands bound with white cord. 

Smoke rose from a building nearby. The bodies of three Iraqis lay on one side of the road, covered with blankets. Another body was on the other side of the road. 

Lt. Belcher said the Marines uncovered "weapons caches throughout the city." And, chillingly, Marines also found Iraqi gas masks still sealed in plastic and displays showing what to do in the event of a chemical or biological attack. 

Elsewhere in central Iraq, somewhere between the cities of Diwaniyah and Kut, thousands of Marines headed out on a route so recently secured that at one spot, the bodies of four Iraqi soldiers were sprawled around their vehicle - which still had its engine running. It looked as if the vehicle had taken a hit from the air. 

Heading north, the terrain was greener and lusher. Marines were entering towns, where in many cases they were greeted by Iraqi civilians smiling, waving and selling Iraqi-brand cigarettes. 

Southeast of Baghdad, Marines seized the strategic town of Kut and routed the Baghdad division of the Republican Guard, guarding the highway to the capital. "The Baghdad Division no longer exists, and the 1st Marine Expeditionary Force is moving on," said Navy Capt. Frank Thorp, a U.S. Central Command spokesman. 

To the west, lead elements of the Army's 3rd Infantry Division pushed through a gap west of Karbala after a night-long bombardment of the Shiite holy city some 50 miles from Saddam's seat of power. 

The division's First Brigade captured a bridge over the Euphrates River at Mussayib, about 40 miles southwest of Baghdad. The bridge had been rigged with explosives, but engineers defused them. 

However, the division's advance slowed to a crawl Wednesday night because inaccurate maps showed existing routes, creating a miles-long traffic jam as thousands of troops and vehicles lined up to cross the Euphrates. 

Eventually, the U.S.-led forces intend to launch a synchronized attack on Baghdad with the infantry, the Marines and the Air Force, said Thorp. 

Their efforts could be hampered by the weather, with temperatures expected to climb into the 90s over the next several days. 

In northern Iraq, meanwhile, U.S. forces received 40 nearly combat-ready vehicles Wednesday in a shipment via Turkey, despite Turkish insistence that the U.S. military cannot use its territory as a supply corridor for weapons. 

A convoy carried food, fuel and medicine for U.S. troops - items approved in a hard-won agreement between Washington and Ankara. 

The convoy also carried 40 pickup truck-style Defender 110 Land Rovers, which had poles in the open rear carriage that apparently could serve as mounts for machine guns. No weapons were seen in the shipment.