Crackling small-arms fire clanged off the damaged vehicle from an adjacent house. U.S. soldiers answered with increasingly violent volleys -- .50-caliber machine gun bursts, tank rounds and a TOW missile -- but the shots from inside the house kept coming. Finally, an ear-splitting succession of five rounds from the tank's big gun reduced the building to flaming rubble and lit the empty streets with white sparks from exploding power transformers.

In the largest urban assault since the siege of Fallujah last November, more than 5,000 U.S. and Iraqi troops entered this northern city before dawn Friday. But the 45-minute firefight at day's end suggested that the insurgents who have controlled much of Tall Afar for almost a year would not relinquish it easily.

"We knew they were going to fight," said Pfc. Johnny Lara, a machine gunner from Blue Platoon, Eagle Troop, 2nd Squadron of the Army's 3rd Armored Cavalry Regiment, who watched the clash with a reporter from a rooftop about 100 yards away. "Now it's a fight."

During the course of the day, at least 30 insurgents were killed as U.S. troops conducted house-to-house searches in the baking sun. Apache attack helicopters that circled the city of 250,000 all day killed 27 people, including eight who were attempting to conceal roadside bombs in old tires, commanders said. No American or Iraqi army casualties were reported.

Set on an old smuggling trail that winds though pastoral plains about 40 miles from the Syrian border, Tall Afar is a key logistics hub for insurgents operating across northern Iraq, military officials say. Like the string of towns a few hundred miles to the south in Anbar province, where Marines have launched a half-dozen offensives since early May, Tall Afar is considered a staging point for operations essential to sustaining an insurgency, such as trafficking of men and arms and providing safe accommodations for fighters.

One year ago this month, U.S. and Iraqi forces stormed into the city after a series of roadside bomb attacks on their supply convoys. But soon after that offensive -- in a pattern repeated elsewhere in Iraq -- the bulk of the troops withdrew from the region, leaving about 500 behind to police a vast swath of northwestern Iraq, including Tall Afar and a more than 100-mile stretch of the Syrian border.

By the end of October, the insurgents had returned, stronger than ever and with more foreign fighters backing them. They quickly reasserted control over the city through intimidation -- kidnappings and beheadings -- and a highly effective campaign aimed at persuading Tall Afar's majority Sunni Turkmens that the U.S. operation was directed at them.

"The September operation basically made people angry, which the insurgents were able to take advantage of," said Maj. Bob Molinari, 35, of Fort Carson, Colo., the planning officer for the 3rd Armored Cavalry, which was shifted from Baghdad in late April as the situation here deteriorated. The offensive "had the opposite effect than was intended. We created a power vacuum and they filled it."

All but a few dozen local police officers quit, many siding with the insurgents. The city's imams and schoolteachers were replaced with newly arrived adherents of radical Islamic sects, U.S. military commanders and local residents said. After a year of violence, as many as half of Tall Afar's residents have fled to outlying areas, leaving behind a ghost town of shuttered shops and charred hulls of vehicles.

"This place is not a town, it's a cemetery. It is the lowest of the low in Iraq," Najim Abdullah Jabouri, a former general in Saddam Hussein's army, said in an interview the day before the operation began. "It needs to be cleaned out."

Jabouri was brought here from Baghdad by U.S. and Iraqi forces four months ago to serve as Tall Afar's police chief.

Col. H.R. McMaster, commander of the 3rd Armored Cavalry, said Tall Afar's complex demographics would make it difficult to pacify. As many as 75 percent of residents are Sunni Turkmens, many of whom held prominent political and military positions when Iraq was ruled by Hussein. Increasingly threatened by the rise of the country's Shiite-led government, they have clashed with local Shiite Turkmen tribes and with the mostly Shiite and Kurdish security forces deployed to Tall Afar.

The Turkmens' ethnic ties to neighboring Turkey have also complicated matters for U.S. forces. During last year's invasion of Tall Afar, Turkish Foreign Minister Abdullah Gul threatened to suspend cooperation with the United States on matters related to Iraq.

"The city is basically a microcosm of all the problems, all the divisions that exist in Iraq, in one place," McMaster said.

Soon after arriving four months ago, the 3rd Armored Cavalry reached out to local tribes of Sunni and Shiite Turkmens, encouraging them to bridge their differences and focus their energies against insurgents. The Americans also launched raids in a string of towns ringing Tall Afar, including Afgani to the north, where insurgents were thought to have fled during last year's offensive.

Fighting in the city grew more intense throughout the summer. In May, a car bomb detonated near a Shiite funeral procession, killing at least 15. Insurgents developed more sophisticated defenses against attacks from the air, using a network of cell phone calls to alert gunmen to incoming helicopters. They also used flashlights on the main supply route into the city to signal roadside bombers as convoys approached. At least 13 U.S. soldiers have died here since, including three in the past week.

In recent meetings, McMaster said, tribal leaders implored the Americans to invade Tall Afar again, but this time not to leave so quickly.

"We are trying to learn from the mistakes that have been made here in the past," said McMaster, who takes great pains not to criticize commanders who preceded him here, saying they were handicapped by limited resources and manpower.

McMaster bristled at any comparison between Friday's operation and November's assault in Fallujah, a city roughly the size of Tall Afar but which likely had many more insurgents present when U.S. forces invaded. Despite the fact that many of them were killed, Fallujah became a rallying point for guerrillas, he said, as they blamed the Americans for the destruction of the city.

"I don't want to kill this city, I want to bring it back to life," McMaster said. "We are taking steps to minimize destruction. I want to do it right."

That effort began in full on Friday, as soldiers piled into the back of their Bradley Fighting Vehicles just after 6 a.m. and rumbled onto Tall Afar's eerily quiet streets. Working in squads of 10 to 15, they burst into homes, many of which were abandoned, and interrogated residents whom they did find about insurgent activity. They often leapt from roof to roof to avoid streets they feared were rigged with bombs.

Capt. Noah Hanners, Blue Platoon's commander, interviewed at least two dozen men of military age and checked their names against a blacklist maintained by the regiment. Each man said he was aware that insurgents operated in the neighborhood but did not know who or where they were.

"All we want is to be safe here," said a middle-aged man in a dark gray robe, as his wife and 11 children sat on nearby mats, his wife weeping, his daughter's hands trembling.

"That's what I want, too," Hanners said. "That's why I need your help."