OUTSIDE BALAD, Iraq -- Earlier this month, Shiite Interior Ministry commandos and their Shiite militia allies cruised the four-lane hardtop outside the besieged city of Balad, trying to stave off retaliation for a deadly four-day rampage in which they had all but emptied Balad of Sunnis.

Sunni insurgents pouring in to take that revenge patrolled the same highway, driving battered white pickups and minivans, their guns stashed out of sight. Affecting casualness, more Sunni men gathered on rooftops or clustered on the reed-lined edge of the highway, keeping an eye on the Shiite forces and the few frightened civilians who dared to travel the highway past Balad.

What brought this Tigris River city north of Baghdad to this state of siege was a series of events that have displayed in miniature the factors drawing the entire country into a sectarian bloodbath: Retaliatory violence between Sunnis and Shiites has soared to its highest level of the war, increasingly forcing moderates on both sides to look to armed extremists for protection.

The Shiite-led government's security forces, trained by the United States, proved immediately incapable of dealing with the sectarian violence in Balad. In many cases, they abetted it, residents and police said.

More than 20,000 U.S. troops are based within 15 miles of Balad, but, uncertain how to respond, they hesitated, waiting for Iraqi government forces to step up, according to residents, police and U.S. military officials.

And all that was left holding Balad, and Iraq, together -- the desire for peace and normality still held by the great majority of Iraqis, and the generations of intermarriage and neighborliness between ordinary Shiite and Sunni Muslims -- was ripping apart.

"The people of Balad should not kill the Sunnis who are among them," said one slightly built Shiite man, fleeing his home on the outskirts of Balad. He and 13 women and children of his family were crammed into a single, battered Toyota sedan, stranded by a flat tire near the highway turnoff to the city. "Our relations are not of months or years. It's since the beginning of time," he said. "This relationship has been destroyed in a second."

The principals involved give a straightforward timeline of how that happened.

The trigger event, U.S. and Iraqi officials said, was the killing of two or three Sunni men from the area earlier this month. One of the men had been a local leader of the insurgent group al-Qaida in Iraq, according to Maj. Gen. William B. Caldwell, a U.S. military spokesman.

On Oct. 13, Sunni insurgents took their revenge. In Duluiyah, a Sunni hamlet four miles and across the river from Balad, insurgents kidnapped and beheaded 17 Shiite laborers who had come to work in the date palm groves there. The U.S. military later arrested two Sunni police officers from the town for alleged involvement in the deaths.

Al-Qaida in Iraq fighters instigated the killings, then stood by as innocent Sunnis were killed in the retaliation that followed, said police Maj. Hussein Alwan in Duluiyah.

Hours after the beheadings, outraged and frightened Shiite elders of Balad telephoned an office of Shiite cleric Muqtada al-Sadr in Kadhimiyah, a Shiite neighborhood of Baghdad. Mr. Sadr leads the Mahdi Army, the most feared Shiite militia in Iraq.

Balad's Shiite leaders asked for protection by the Mahdi Army, and for the militia to exact revenge, Taysser Musawi, a Shiite cleric, later recounted.

The Kadhimiyah branch of the Madhi Army responded, Ammar Joda al-Musawi, a spokesman for the militia brigade, said at his office in Baghdad.

"It was like an SOS call," Mr. Musawi recalled. "To protect them from being killed by the Salafis who are killing followers of the prophet" Muhammad, he added, referring respectively to the Sunni insurgents and the Shiites.

Mahdi Army fighters in plain clothes crowded into two buses and headed to Balad, Musawi said. More Mahdi Army fighters followed in army uniforms and army vehicles, Musawi said. Others wore the blue-and-white camouflage pants that Iraq's Interior Ministry commandos wear, but with black T-shirts to distinguish them from the real commandos.

By early in the day Oct. 14, a Saturday, the Shiite forces had assembled to rid Balad of Sunnis.

Mosque loudspeakers blared warnings for all Sunnis to leave the city within 48 hours, residents recalled. Gunmen in uniforms and civilian clothes took control of Balad's streets and outlying roads, police and residents said.

The Shiite gunmen set up checkpoints, quizzing occupants of each passing vehicle about whether they were Shiite or Sunni.

A dentist from Balad, who gave her name as Um Mustafa or mother of Mustafa, lost her husband at one such checkpoint. The Sunni couple and their two young children had tried to flee the city at 7 a.m. that Sunday. But armed Shiites in black were waiting at one checkpoint.

The Shiite gunmen bashed her husband in the face with the butts of their rifles, Um Mustafa recalled, and stuffed him in a sport utility vehicle. She later found his body at the morgue.

By the end of Saturday, the U.S. forces had learned about the mass killings under way in Balad, Gen. Caldwell, the military spokesman, said in Baghdad. A platoon-size quick-reaction force was dispatched that same day, he said.

The U.S. soldiers asked the Balad officials whether they wanted help, Gen. Caldwell said, but the officials declined the offer. The Iraqi government made no request for assistance, he said.

By Sunday, Oct. 15, U.S. forces had received reports of at least 57 people slain.

The Balad morgue had received about 80 bodies by Tuesday, Oct. 17, hospital officials said. Most were Sunnis, and all had been shot; some bore the holes of electric drills.

The Iraqi government ordered in national police commandos, whose forces often have been accused of working with the militias to kill Sunnis.

Forty-eight hours after the attacks on Sunnis started, the Iraqi government ordered in the Iraqi army's 3rd Regiment, 4th Division.

Residents credited the Iraqi army forces, many of them Sunni Arabs and Kurds, with finally quelling the violence.

By then, however, very few Sunnis were left in Balad.

Munthir Lattif, 27, who stayed in the city, called around and found only five or six other Sunni families still in Balad, he said. "We are living in a very difficult situation," he said in an interview at his home. He, his wife and their four children slept on the roof of their home at night, ready to jump to neighboring rooftops if the Shiite gunmen came to their door.