It was the low point of the war for the two generals.

On March 27, outside the city of Najaf, Lt. Gen. William S. Wallace, commander of the U.S. Army's V Corps, met with Maj. Gen. David H. Petraeus, commander of the 101st Airborne Division. As they sat on gray folding chairs in the desert wasteland, the war seemed to be in dismal shape.

The critical crossroads city of Nasiriyah had degenerated into a shooting gallery for U.S. convoys. An Army maintenance unit was ambushed on an overextended supply line. In just one day, 36 U.S. soldiers and Marines were killed, taken prisoner, or missing. Before dawn the next day, the first deep strike by AH-64D Apache attack helicopters was beaten back by small-arms fire that downed one chopper and riddled 33 others with bullets. Then a harsh sandstorm swept in, grounding U.S. helicopters, jamming some weapons, bringing most operations to a halt and demoralizing the troops. And they had not yet engaged the Iraqi Republican Guard, which they expected would greet them with chemical weapons.

Wallace, wearing cotton cavalry gloves and Wiley-X sunglasses, intimated in an interview after the meeting with Petraeus that, in light of the damage sustained by the Apaches earlier in the week, U.S. commanders were reconsidering their tactics. He added, "We're dealing with a country in which everybody has a weapon, and when they fire them all into the air at the same time, it's tough."

Just 13 days later, Baghdad fell.

What ended as a military victory that toppled the Iraqi government in 21 days was filled with moments of uncertainty, miscues and unexpected successes for U.S. forces. This article is an anatomy of the war as described by dozens of military officials and commanders, including key participants in the decision-making on the battlefield and in Washington. They provided an inside look at a conflict that upended a host of specific assumptions about how the war would unfold even as it delivered the final collapse of Iraqi resistance that commanders had forecast.

Some of these participants said the war got off to an unexpected and confused start. But it reached a swift conclusion in Baghdad in part because of the debilitating impact of air power against Iraq's Republican Guard divisions.

In particular, they said, a Special Operations campaign to guide bombing attacks against Iraqi forces even in the midst of a howling sandstorm appears to have been far more effective than generally realized.

But another Special Operations effort, to persuade Iraqi forces to surrender at the outset of the campaign, was suddenly overtaken by the decision of Gen. Tommy R. Franks, the overall war commander, to start the ground offensive a day earlier than planned. This decision, the commanders and officials said, sparked a roiling argument within the military's elite Special Operations units about whether the start disrupted the surrender plan. Some officers say the course of the war would have been far smoother, with fewer casualties, had they been allowed to bring the surrender appeal to fruition.

Despite the successful drive to Baghdad, some commanders still believe the invasion force was too small, and that their supply lines were so stretched that there was a chance that front-line units would run out of food and water.

Finally, officers and Pentagon officials said that during the critical second week of the war, when the two generals met outside Najaf, a sharply different assessment of the state of the war emerged between the field commanders and officials in Washington.

A Sudden Start

The war plan was developed during more than a year of argument among Defense Secretary Donald H. Rumsfeld, Franks and other top officers. It had more gambles built into it than is usual for the U.S. military, officials said, and it was designed to encourage military leaders from Franks down to take chances.

The size of the invasion force was reduced through a series of high-level decisions, although details are still in dispute. One senior Army planner said that two units -- the tank-heavy 1st Cavalry Division and the fast-moving 3rd Armored Cavalry Regiment -- were dropped from the war plan without much explanation last winter, paring the force to a size so small that it worried a few top Army generals. Two Pentagon officials disputed that assertion.

The second big cut in the invasion force came just last month, when a diplomatic impasse with Turkey over the movement of U.S. forces through its territory led to the 4th Infantry Division being removed from the initial invasion force. That was the first pivotal moment in the war. Franks could have asked for time to wait for the 4th Infantry Division's armor to get to Kuwait, but did not. "The only comment that was made was, 'We don't have the 4th, it will be harder,' " according to one Army general who was involved in the discussion. Far more controversial, especially among field commanders and Special Operations officers, was Franks's next move: advancing the date of the ground invasion to Thursday, March 20, and launching it without a preliminary bombing campaign against Iraqi troops.

The order to abruptly start on that night caught senior U.S. commanders by surprise and may have doomed their plans to persuade Iraqi army units to capitulate without a fight. Although the U.S. military had been dropping leaflets on the Iraqis for weeks, warning them not to fight, only that day did they start dropping leaflets that gave specific instructions for how to capitulate -- turn the turrets of their tanks and artillery around, put their vehicles in a square, stay at least 1,000 feet away from their weapons and hoist white flags.

"We weren't able to get the message out," said Lt. Col. George Smith, a top Marine war planner who had been drawing up schemes to defeat Iraq since January 2002. "If they got the message it, was probably right before ground forces were upon them."

The Marines had hoped that an early capitulation by Iraq's 51st Mechanized Division, sitting right on Iraq's border with Kuwait, would set the tone and encourage other Iraqi units to quickly follow its example. Those efforts "didn't bear fruit," said Lt. Gen. James T. Conway, commander of the 1st Marine Expeditionary Force. "I don't think the snowball got rolling properly."

The U.S. military had expected mass surrenders on the order of what it saw in the Persian Gulf War a dozen years earlier. Planners forecast as many as 64,000 Iraqi prisoners of war. As of today, they had about 6,200.

A Special Operations officer working in the region said the early move also undercut crucial intelligence-gathering efforts by hundreds of Special Operations troops who quietly slipped into Iraq on March 20 to join CIA and military operatives who had been inside Iraq for weeks.

The officer said the short time between S-day, when the Special Operations troops moved into Iraq, and G-day, when ground forces moved, "severely impacted" the ability of the advance guard "to build rapport with opposition groups and conduct unconventional warfare with them." He argued that this change in plan helped lead to the initial success of Saddam's Fedayeen, the militia fighters whose persistent attacks on U.S. forces moving through the south proved to be a major surprise of the war.

"The end result was the Fedayeen was effective because the unconventional warfare effort did not have time to identify them and neutralize them," the officer said. In addition, he said, the Fedayeen's attacks signaled to Iraqis in the early days that their government might fight and survive. "That small illusion of hope greatly impacted the psy-op [psychological operations] campaign," the officer recalled.

The early start also meant that the commanders had to battle troops who had not been attacked first by airstrikes, or "shaped," as military planners put it. It also meant they would not wait for more ground troops to arrive, leaving the Marines and the Army's V Corps with exceptionally large areas of operations to cover with relatively few men.

A Bad Week

The U.S. invasion force moved quickly once it was launched, with the Marines charging into the big Rumaila oil field in southern Iraq, to keep it from being set afire, and the Army's 3rd Infantry Division rumbling northward more than 150 miles across the Iraqi desert.

But the war turned ugly on the first weekend, March 22 and 23. Contrary to expectation, the Shiite Muslims of southern Iraq did not rise up to greet the advancing U.S. troops as liberators. Instead, it seemed at times to U.S. troops as if many people in southern Iraq were waiting for them with an AK-47 assault weapon in hand.

The ferocity of the Fedayeen surprised the U.S. commanders. "Nasiriyah was more difficult than we envisioned," said Col. Larry K. Brown, the operations chief for the Marines in Iraq. "I thought there would be a few of these knuckleheads out there and we would blow past them. It turned out there were more of them and they were more fanatic."

On Sunday, March 23, the worst day of the war for U.S. forces, Iraqis ambushed a convoy of the Army's 507th Maintenance Company that had taken a wrong turn, killing or capturing 12 soldiers. While the Army and Marines were still absorbing the news of those attacks in their rear, worse news came from the front.

In the predawn hours of Monday, March 24,the Apache helicopters of the 11th Aviation Regiment went on a first mission to attack the Republican Guard. The full extent of the disaster from gunfire became known the next morning, when Wallace, the V Corps commander, told his subordinates by telephone that battle damage to the helicopters had made the regiment essentially not battle-worthy.

The significance of this was that the Army's premier deep-attack weapon appeared to be neutered. Just days before, an Army general had boasted about how an Apache battalion armed with Hellfire missiles could destroy an armored brigade in 20 minutes. Instead, the regiment had destroyed virtually nothing and had been thoroughly ventilated for its efforts. U.S. intelligence eavesdroppers had detected 50 cell phone calls in the target area as the Apaches approached, a crude but apparently efficient Iraqi early-warning system. A thrown master switch in one town shut off all the lights, a signal for a barrage of fire that threw up a wall of lead.

Monday also brought the onset of three days of monstrous weather -- high winds and blinding dust that stopped virtually all helicopter flying and in some cases halted all ground transportation at a time when the military was so stretched there was concern the 3rd Infantry Division would run out of water, according to Army sources. Some units were "black" on food, meaning that they were down to within a day or two of empty larders.

The issue of how to secure the long supply lines led some commanders to conclude that the Army needed "more forces, perhaps much more," as one general put it at the time. But the nearest available substantial force -- the 4th Infantry Division -- was still at least three weeks from getting into the fight.

The next morning, Tuesday, March 25, the three top commanders of U.S. ground forces in Iraq convened a grim video teleconference to figure out what to do about their war gone sour.

As Lt. Gen. David D. McKiernan, the overall land commander based in Kuwait, solicited recommendations, some of the generals were pessimistic about continuing to Baghdad without first securing overextended supply lines and cleaning out fierce pockets of resistance in the south. One Marine recalled that, at the meeting, one Army general expressed doubt that U.S. forces were ready for the push to Baghdad. Conway, the senior Marine commander in Iraq, argued that in just five days, the U.S. ground force had seized Iraq's southern oil fields, captured two critical bridges over the Euphrates River and raced up the road toward the capital. "This action has come at a cost, and that cost has been casualties," conceded Conway, the officer recalled. "But as a result of this action, we are now ready for the push to Baghdad."

By March 26, in the third day of the sandstorm, concern had turned to anxiety among the commanders. The 3rd Infantry Division had two M1 Abrams tanks and a Bradley Fighting Vehicle hit hard. It was the first time an Abrams had ever been lost in combat, and provoked concern that the Iraqi military had a better antitank capability than realized.

A plan to drop a brigade of paratroops from the 82nd Airborne Division into the Karbala Gap, a key position on the road to Baghdad, was scrubbed, in part because the drop zone was found to be very rocky and because they could not be reinforced quickly. At least one commander concluded that pulling back and consolidating the U.S. position in Iraq might be necessary.

The two-lane blacktop on which all resupply was moving was code-named Boston, and it was the primary land link to bases in Kuwait. There had been ambushes farther south, and if the Iraqis had attacked Boston "once or twice, they would have brought the whole thing to a screaming halt," one general said.

On Thursday, March 27, Wallace came to Petraeus's desert command post for their somber meeting on the folding chairs. It was the first time in three days that it was really possible to sit outside. On the front lines in Iraq, recalled one Army battalion commander, "It seemed like the whole world was going crazy."

View From the Pentagon

The battlefield looked much better from the Pentagon. "There was a good feeling about how the war was going" at that point a week into the war, a senior Defense official recalled today. The official toted up the gains of the first week: The southern oil fields had been secured, and more than 1,000 Special Operations troops had moved from Jordan into the western Iraqi desert, shutting down much of the threat that Iraq might be able to launch Scud missiles or drone aircraft laden with chemical or biological weapons. U.S. forces were only 50 miles from Baghdad.

Also, Pentagon officials concluded the bombing of the Iraqi military had been far more effective than those on the ground realized. Even during the sandstorm, the Republican Guard's Medina Division, keystone of the Iraqi defenses ringing Baghdad, was being pounded around the clock by U.S. aircraft. Two Special Operations teams on either side of the Medina positions in the Karbala Gap helped guide the bombing with laser spotters and other devices.

Retired Gen. Ronald R. Fogleman, a former chief of staff of the Air Force, noted that while the Army and Marines paused, Air Force and Navy warplanes were taking apart the Republican Guard. He said that with the expanded radar and spotting capabilities, "air power was able to continue with the battle plan while the ground forces stopped for needed rest and resupply."

Franks originally had scheduled the final drive for Baghdad to begin the weekend of March 29-30, a senior military officer recalled. But before giving the green light, he wanted to see an assessment of the effects of the bombing, and that couldn't happen until the storm passed. He delayed the attack by two days.

"Right after the sandstorm ended, we started getting indications that they were getting pounded," said a senior military officer. And when the Air Force's "bomb damage assessments" finally arrived that weekend, the results were astonishing. The Army had wanted to hold back until the Medina Division was judged to be cut to 50 percent of its original combat effectiveness. Instead, the Medina was assessed to be at just 20 percent.

The result, said a senior military officer, is that the war looked very different to ground commanders than it did to Franks or to his bosses at the Pentagon. "There are real disconnects," he said. But, he added, "I don't think there has ever been a battle where there hasn't been a strategic-tactical disconnect."

The Camp David Talk

The most important meeting of the war may have been the one held on the morning of Saturday, March 29, on a wooded ridge in the Maryland countryside, at the Camp David presidential retreat. Some retired generals were arguing that U.S. forces in Iraq should wait for reinforcement from the 4th Infantry Division, and some Army officers on active duty privately agreed with that view.

Former House speaker Newt Gingrich (R-Ga.), who is close to Rumsfeld and a member of the Pentagon's Defense Policy Board, recalled that the discussion was a turning point. "You had this moment when the old Army was pounding away, saying that we were out there and facing the Republican Guard" with too small of a force, said Gingrich. "That was the moment of optimum danger. A less confident administration might have paused and waited for another division to come up."

At the Saturday morning meeting, held as a video teleconference, President Bush "was not an impatient person," recalled a senior administration official. "He was prepared to let things unfold."

The meeting's conclusion, said a presidential adviser, was that the campaign should remain "Baghdad-centric," and that the forces should push on to the capital as soon as possible, rather than try to secure their supply lines and consolidate their positions in southern Iraq. The thinking, recalled this adviser, "was that if you cut off the head of the snake, the rest of the snake wouldn't be able to eat you."

The president also had another agenda, said this official. Several people close to Bush said the calculated risk of plunging ahead was driven partly by the realization that it was important for Rumsfeld's ambition of transforming the military into a lighter, more agile force. Slowing down on the battlefield threatened to suggest a reversal of the administration's key defense policy.

"The people who were bad-mouthing the plan were the anti-transformation forces, the heavy-Army guys," said this person, who has participated in numerous war-planning meetings. "They wanted 600,000 troops in there. By not waiting around, it had the effect of winning that debate." The message that came down the chain of command from that meeting, said a senior military officer, was, "Stay the course."

The Drive to Baghdad

On the morning of Tuesday, April 1, the Army and Marines moved out.

Maj. Gen. James N. Mattis, commander of the 1st Marine Division, pulled an April Fool's trick on the Iraqis around Kut, the strategic city guarding the eastern approach to Baghdad. The 1st Marine Regiment moved north along Highway 7 to the southern edge of Kut in a feint to make the Baghdad Division of the Republican Guard think that was where the attack would originate. But at the same time, the 5th and 7th Marine regiments that had appeared to be heading for Baghdad instead circled back and slammed into the Iraqi rear from the west, shredding what few units were left after days of airstrikes.

After the battle, Mattis, who has a reputation of being extremely aggressive, abruptly fired the commander of the 1st Marine Regiment, Col. Joe W. Dowdy. As part of the feint, it was Dowdy's job to nudge his troops forward just enough to draw Iraqi artillery fire so that U.S. warplanes could locate and destroy the batteries. Those who were in the combat operations center that night kept waiting for the regiment to draw the Iraqi fire, but it never did, Marine officers recalled.

Another Marine officer said the problems at Kut were "the final straw" in a series of incidents over three days that had caused Mattis to decide that Dowdy should no longer command the regiment. Mattis never publicly explained Dowdy's dismissal, and the colonel has not commented. "He's a great Marine, just not the right man for the time," Conway said later. "The MEF [Marine Expeditionary Force] is better with Joe doing something else."

Perhaps the most chilling moment for the Marines came the next day, April 2.

Before the war began, U.S. planners pinpointed three moments they thought Hussein might resort to weapons of mass destruction: when U.S. forces first crossed the border from Kuwait, when they arrived at Nasiriyah to ford the Euphrates River, and -- most likely -- when they began attacking the Republican Guard and threatening the capital.

That day, military intelligence intercepted a transmission from Baghdad to a senior Iraqi commander believed to have control over chemical weapons. The entire transmission consisted of just one word: "Blood."

Intelligence analysts interpreted that as an order to launch a gas attack against U.S. forces. Wind patterns that night were perfect for a downwind attack. Commanders ordered troops throughout Iraq to wear their chemical protective suits and have their gas masks at the ready. Top officers at Marine headquarters crawled into their sleeping bags that night wearing their suits.

But dawn came and the gas did not. All three moments of critical danger had now come and gone with no attack. Commanders allowed troops to doff their protective suits.

By this point, it became increasingly clear that Hussein's command and control was rapidly disintegrating along with the Republican Guard. "They just weren't able to keep up with our tempo," said Col. Alan Baldwin, chief of intelligence for the Marines in Iraq.

At one point, an Iraqi major general, Sufian Tikiriti, trying to flee Baghdad, ran right into a Marine checkpoint and died in a hail of machine-gun fire when he tried to drive past it. At another point, a giant column of artillery and other Iraqi military vehicles tried to slip out of the city under cover of night, only to run into a blitz from the air, monitored on live reconnaissance video at Marine headquarters, where Lt. Col. David Pere, the senior watch officer there, called out grid coordinates for warplanes to obliterate the next target. All told, about 80 vehicles were hit.

Taking the Capital

No one in the U.S. military really wanted to attack the capital. From the start, the prospect of an urban battle in the warrens of residential neighborhoods in a city of 5 million deeply worried the commanders. No amount of technological wizardry or air superiority could truly counter the risks of heavy casualties in the streets.

One night after outlining a plan to cordon the city and send in probing attacks, Smith, the Marine war planner, stopped and admitted nothing troubled him so much as a fight inside Baghdad. "We're still hoping for the golden BB," he admitted, referring to the long-dreamed-of magic solution when CIA or Special Forces would simply assassinate Hussein.

"It's eerie how close some of the situations are to what we war-gamed," another officer said at the time. "We have even done vignettes all the way to Baghdad. And then it gets too difficult and we give up."

The first task was to ring the city and see what happened, but for the first time in the campaign, the Iraqis blew up some bridges to slow the U.S. advance. When Marines arrived at the Diyala River, a small tributary of the Tigris that runs along the eastern edge of Baghdad, they had to summon engineers to lay down pontoon bridges and to erect their own steel span at a crossing destroyed by the Iraqis. As the Army began stabbing into the center of Baghdad, the Marines were itching with impatience.

"It was getting to be a little frustrating to me, I will admit, that we had crossed two major rivers without interruption, the Euphrates and the Tigris, and this 60-meter Diyala stopped this mighty MEF cold for 20 hours," Conway admitted later.

But if the Marines were not the first to enter the city, they ended up with the most memorable moment. At 11:30 a.m. on Wednesday, April 9, McKiernan, Wallace and Conway got together by video teleconference to try to settle on a plan to assault Baghdad. The Army would drive in from the west while the Marines would charge in from the east and they would meet in the middle. By this plan, they would capture the city in four days.

By 6:30 p.m., the generals convened another teleconference. Events had outrun their plan. Hussein and his top cronies had vanished, and the government of Iraq all but disappeared. The Marines, with help from Iraqis, yanked down a statue of Hussein.

Ricks reported from Washington. Staff writer Mike Allen in Washington contributed to this report.

Maj. Gen. David H. Petraeus, right, commander of the 101st Airborne Division, confers with Lt. Gen. William S. Wallace, V Corps commander, on the first day of the ground invasion.Tank crews from the 4th Battalion, 64th Armor Regiment, wave pistols in the air in a "Seminole Indian war dance" before moving into position to cross the border from Kuwait into Iraq. Army Spec. Edgar Hernandez, of Mission, Tex., is shown on Iraqi television after he was captured near Nasiriyah. His unit of the 507th Maintenance Company was ambushed.Marines with 3rd Battalion, 7th Regiment, take cover from an Iraqi mortar attack on a road south of Baghdad during a fierce sandstorm that stalled the U.S. ground offensive.After a three-week ground offensive, Marines of the 3rd Battalion, 4th Regiment, take positions at a main intersection in downtown Baghdad as Iraqi defenses collapsed.