But part of the price for the rapid advance was Iraq's first two successes in shooting down U.S. aircraft. In fighting near Karbala, 50 miles south of Baghdad, Iraqis on Wednesday downed an Army Black Hawk helicopter. The crash of a Navy F/A-18C jet was still being investigated Thursday, though it also appeared likely it was downed by enemy fire, officials said.
Six Americans aboard the helicopter were killed, they said.
On Thursday, large sections of the Iraqi capital lost power as loud explosions were heard on the outskirts of the city.
The explosions persisted for nearly 15 minutes before the power went off — the first widespread electrical failure in the city since U.S. bombardment began two weeks ago. The reason for the loss of power was not immediately clear.
It was the first time that significant portions of Baghdad have lost electricity since the U.S.-led bombardment began March 20.
Meanwhile, lead units of the multi-pronged U.S. Army and Marine assault force were on the outskirts of the edge of Baghdad.
Paving the way, special forces infiltrated some Iraqi command posts in the Baghdad area. Another group of commandos raided the Thar Thar presidential palace, in a resort area about 55 miles from Baghdad.
Brig. Gen. Vincent Brooks, in a briefing at Central Command, said the raiders landed by helicopter in the palace compound after suppressing anti-aircraft fire. The commandos found no leaders in residence, but came away with documents that will be reviewed by intelligence officers, Brooks said.
U.S. officials said only one palace was raided; Navy Capt. Frank Thorp, a Central Command spokesman, initially indicated soldiers also had entered a palace near Baghdad's Saddam International Airport.
Special forces infiltrated some Iraqi command posts in the Baghdad area during the night, seeking strategic information, and also secured some bridges and dams to forestall possible sabotage, according to the U.S. Central Command.
The advance set the stage for either a final push on the capital or the capitulation of Saddam's best and most loyal fighters. Some Pentagon officials said Wednesday the American forces likely would pause on the outskirts of the capital to allow pressure to build on the Iraqi regime, perhaps enough so it would fall without the chaotic and bloody urban fighting Iraqi officials say they are planning.
A pause also would allow more reinforcements to enter Iraq. The 4th Infantry Division, which has some of the Army's most advanced tanks and equipment, is arriving in Kuwait and could field a brigade-sized task force of a few thousand soldiers as early as Monday or Tuesday, the official said.
"They may stop and do some probing, but I think it's very unlikely they'll wait for the 4th Infantry Division," says CBS News Analyst Perry Smith. "They've got the momentum, they might as well go forward.
"They might not go directly to the middle of the city. One of the key places they want to grab is the International Airport."
Once this is accomplished, Smith says, "We can start flying stuff in which could be very, very helpful."
Still, the Pentagon sought to lower expectations that the Iraqi capital could be taken quickly or easily.
"We are planning for a very difficult fight ahead in Baghdad," Maj. Gen. Stanley McChrystal told a Pentagon news conference. "We are not expecting to drive into Baghdad suddenly and seize it."
If American forces do battle to take Baghdad, whether after a pause or not, the strategy is likely to include cordoning off the city and targeting key sites for attack, a military official said.
The Americans would try to avoid the street-by-street battles that Iraq wants to set as traps by focusing on such key areas rather than trying to take over the entire city, the official said.
"It might well collapse, but there are some hard-core people left in Baghdad we're going to have to worry about," says Smith.
The U.S. lost its first confirmed aircraft Wednesday -- shot down amid the rapid advance. Pentagon officials said it was downed by small-arms fire. There was no word early Thursday on the fate of the pilot of the Navy plane.
In Kut, an Iraqi military town on the Tigris River southeast of Baghdad, Marines from the 3rd Battalion, 4th Marines were battling Iraqi fighters building to building.
The Marines were jumping on rooftops and going through all the rooms in some buildings. Tanks roamed the city and the Marines were taking small arms fire and mortars. Three Marines were wounded, two lightly, one more seriously.
On the outskirts of Kut, Marines opened fire on a military training academy, blowing a hole into a mosaic portrait of Saddam.
The temperature was to reach into the 90s Thursday and U.S. troops were expected to remain in chemical protective gear after unconfirmed reports that Iraqi commanders had given orders to use chemical weapons when coalition forces crossed the bridge.
So far no such weapons detected, but the troops were hot and miserable.
One scout vehicle was hit by a rocket-propelled grenade.
"A scout just got hit by an RPG, this is serious up here," said Capt. Chris Carter from Wakinsville, Ga.
The Marines southeast of Baghdad began the Thursday drive toward the capital protected by withering artillery and mortar fire. A massive convoy of moved along the main road leading to the Iraqi capital.
The U.S. convoy was kicking up huge plumes of dust as the hundreds of vehicles rolled down the single tarmac and dirt road. All the talk on the radio was about how to clear up the traffic jams.
U.S. military officials have weathered criticism over the past week as commanders, other Pentagon officials and outside analysts said the war effort had been slowed by unexpectedly stiff resistance and a plan that may have relied on too few troops.
McChrystal and Victoria Clarke, spokeswoman for Defense Secretary Donald H. Rumsfeld, told reporters at least six times in a 30-minute briefing that the toughest fighting may lie ahead.
U.S. commanders would be pleased if their reception in Baghdad resembled the scene Wednesday in the southern city of Najaf, where American journalists reported that thousands of Iraqis poured into the streets to welcome a Humvee column carrying an American colonel and his troops.
The Los Angeles Times reported that the Americans had managed to secure Najaf's gold-domed Ali Mosque, one of the most sacred sites for Shiite Muslims, after three days of combat. The Americans said some of their troops had been fired on by Iraqi forces inside the mosque.