"Peace be upon you," they said, and Abu Salam, a man in his 50s, repeated the greeting.

The pair, one fat and the other thin, spoke politely. Both were clean-shaven and wore slacks and button-down shirts.

"You are now aware the neighborhood of Muwallamin belongs to the Islamic State of Iraq," the bigger man said. "We have three conditions you can accept: You can pay a tax, become a Muslim, or you can leave your house and we will help you take out your furniture."

"We'll let you make up your mind."

"Peace be upon you," the men repeated as Abu Salam watched them head back toward the street.

Within hours, Abu Salam and his family left their neighborhood of more than 50 years. They joined an exodus that has all but emptied Dora, a large district in south Baghdad, of its once-thriving Christian population.

Abu Salam, who spoke on condition that his real name not be used, citing fears for his safety, is staying elsewhere in Baghdad for now.

"People will leave if things don't get better. It is chaos," he said. "If there is no imminent solution, Iraq is finished."

Christian leaders say 500 families left Dora in April and May. The US military concedes that a large number of Christians were uprooted but says the number is not that high. The United Nations' refugee agency said it counted at one location 100 families that had fled Dora.

The flight of Dora's Christians is an example of how the initial phase of the US security crackdown has failed to establish security and stop sectarian purging in Baghdad's neighborhoods.

The US military conducted a major clearing operation in Dora last fall, then largely pulled out, turning security over to Iraqi forces. Sunni Arab militants with ties to Al Qaeda in Iraq quickly reestablished themselves and late last year began harassing Christians. A second US sweep in early winter failed to loosen the militants' grip on the district.

In interviews, displaced Christians described a civilian population too terrified of Al Qaeda to ask Americans for help. They said that even after the Baghdad troop buildup started in February, US soldiers were rarely present in some neighborhoods and often had no idea what to look for.

Major Kurt Luedeke, the US military spokesman for Dora, said US officials were caught off guard by the campaign against Christians in the area.

"We knew it was going on; we just didn't know how widespread it was," he said.

Iraq's senior Christian politician, Younadam Kanna, said the military didn't launch an offensive against militants in Dora until May 25, although the campaign to drive out the district's Christians had begun in earnest in late April.

"There weren't enough forces," Kanna said. "The multinational forces are isolated from the people. . . . They don't know who is who."

In response to the mass displacements, the US military has strengthened its presence in the area. In a bid to contain Al Qaeda supporters and prevent further neighborhood purges, the US Army has erected concrete barriers and walled off certain streets.

Troops are surveying every home, collecting photographs, fingerprints, and retinal scans of all military-age males. With the additional troops, the US Army says it can patrol every neighborhood in Dora, sometimes several times a day.

The Christian community's troubles in Dora began in autumn 2004, when Sunni militants bombed churches and kidnapped people. But Christians' lives took the sharpest turn for the worse in October after Al Qaeda and allied groups declared an Islamic State of Iraq.

By January, the Islamic State's proclamations appeared on walls and were circulated in leaflets. Dora residents said some of the fliers proclaimed that women were required to wear veils; shorts were banned; trousers were forbidden for men; and cellphones were prohibited for women.

"They issued laws and decrees like a real state," said Wardiya Yussef, who left Dora in April after a cousin was shot on the street.

As Al Qaeda supporters asserted their muscle, Iraqis were reluctant to give information to the Americans. It was a bitter reversal from the period last fall when military sweeps in Dora had briefly restored stability. In that regard, too, Dora fits a broader pattern in which civilians have watched US troops come and go, seldom staying long enough to establish lasting security.

That lack of constancy has bedeviled US efforts in Iraq, said Stephen Biddle, a military analyst at the Council on Foreign Relations.

"Until local civilians come to believe that you'll be there long enough to protect them from reprisals, and that you're stronger than the militants . . . they won't trust you well enough to risk offering tips and information."