The plan for the day was to keep chipping away at President Saddam Hussein's power structure. The Marines would stab into Baghdad and seize a paramilitary base, a secret police headquarters and a presidential palace. They would drive around, but not through, Saddam City, the Shiite sector where intelligence indicated Hussein still had a tight grip.

Then new intelligence arrived -- via CNN. As he studied the television in his trailer here to the east of Baghdad, Lt. Gen. James T. Conway saw dramatic pictures of Iraqi Shiites in the streets of Saddam City celebrating Hussein's impending fall. Hussein, clearly, had no hold on the area.

Time to send in the tanks.

In the military, as in football, it is known as calling an audible. They had been calling a lot of audibles in the last few days here at the desert headquarters of the Marine commander in Iraq. The dizzying pace of events unfolding in Baghdad caught U.S. military strategists by surprise. Beset by poor intelligence in the city and finding less resistance than expected at every turn, the generals scrapped their plans and ordered their troops to keep going.

The ad hoc decision to send the Marines all the way into the heart of Baghdad, where they assisted in toppling a statue of Hussein and helped erase his power, culminated a drive for the capital that confounded expectations again and again. The bloody Stalingrad-style battle some feared never materialized. While sporadic fights continued even today, organized opposition vanished. Even the Special Republican Guard, Hussein's praetorian guard, appeared to flee or melt into the population.

Yet in the exuberance of the moment, U.S. generals worry that the pictures on television might be misleading. Although Hussein no longer rules Baghdad, a city of 5 million, neither yet do U.S. forces. Looting raged out of control, and small-scale attacks on U.S. troops remain likely for some time. And more battles may loom on the horizon, particularly in Tikrit, Hussein's home town about 90 miles to the north.

"I don't think that Baghdad has by any means fallen," warned Conway, a three-star general who leads a force of more than 85,000 Marines and British troops. "I'm encouraged by recent developments, certainly, but I still think we're some days, I hope not weeks, away from Baghdad being secure."

To be secure, Conway said, the Marines and Army must have freedom of movement throughout the city even in small units, begin to meet with civic leaders who can start to establish a new Baghdad, be able to reinstate basic services such as power and restore "a basic semblance of law and order."

As looting ran rampant today, though, Conway appeared largely unbothered.

"We would have to stop it," he said in an interview. "But I think I understand it, if you look at how these people have been deprived over the years. This is a potentially very rich country, and you can argue that this national wealth has been looted by this whole regime for over a couple decades now. Does it bother me now that I see these people taking an office chair or the guts of a refrigerator or an air conditioner? Not really, not at least at this point. Because in some regards it's got to be the pent-up frustration they've experienced for the better part of their lives."

But the disorder threatens to complicate U.S. hopes of quickly addressing humanitarian needs in Baghdad. The International Committee of the Red Cross beseeched the U.S. military to come to their aid at three buildings in Baghdad besieged by looters -- its main headquarters, a psychiatric hospital and a warehouse.

Army Lt. Gen. David D. McKiernan, the top U.S. land commander, based in Kuwait, ordered the Marines to dispatch troops to those locations to help, according to officers here. "We can't make the mistake to say that's happening all over Baghdad," Conway said of the televised street celebrations. "We still have a military imperative to conduct operations to reduce those regime-related facilities."

Managing the civilian population also could be vexing. In eastern Baghdad this morning, a mob of about 1,000 Iraqis who had fled the city the night before almost rioted when Marines would not let them return to their houses, officers said. Only after the Marines reversed course and allowed them back into the city on foot did the crowd calm down.

"All of us need to be steeled for continued high-intensity combat in a huge urban environment which historically produces high casualties," warned Lt. Col. George Smith, a top planner for Conway. "There's still some heavy combat on the horizon, and I think everybody needs to be prepared for that. The remnants of the regime we do not believe are just going to go away."

The day started with U.S. forces focused on tightening the cordon around Baghdad. Wary of a full-throttle assault in a sprawling city with plenty of potential ambush alleys, the Army and Marines had been slowly picking at the city's defenders and trying to cut them off from reinforcements.

Elements of the Army's 3rd Infantry Division and the 1st Marine Division were trying to hook up at Highway 2 north of Baghdad to complete a circle. Other Marine units were assigned to stage raids on selective symbols of Hussein's power on the eastern side of the city: the headquarters of the Directorate of General Security, a Special Republican Guard barracks, a compound used by paramilitary forces and a palace on the banks of the Tigris. By midday, the Marines had already hit all of their objectives.

During the day, the Marines were supposed to drive around the periphery of Saddam City, a massive slum populated by 2 million Shiites on the northeast side of Baghdad. While hopeful that the Shiites would welcome Americans, U.S. commanders had heard that Hussein had clamped down there and did not want to send the Marines inside.

At the same time, Conway joined other generals on a video teleconference at 11:30 a.m. to discuss the endgame for Baghdad. The generals settled on a plan for a decisive assault on the city: This weekend, the Marines would drive in from the east, the Army would push in from the west and by Sunday the Iraqi government would probably collapse.

But as Conway likes to say, "the plan goes to hell the moment you cross the line of departure." As the day progressed, it became clear that opposition had collapsed. The Army found a motor pool with 100 abandoned Iraqi tanks. And then hundreds of residents flocked into the streets of Saddam City, cheering and chanting against Hussein, unafraid of any government security organs.

Conway was struck by the televised images. Within hours, tanks headed for the center of town, Marines wrapped a chain around the neck of a Hussein statue, and a tank recovery vehicle, which normally tows wrecked armor, yanked it off its pedestal.

At 6:30 p.m., Conway and the other generals got together for another video teleconference. Forget the plan, they quickly concluded. That's OBE, Conway said: "Overtaken by Events."

Lt. Gen. James T. Conway says that the "plan goes to hell the moment you cross the line of departure."