While Americans were glued to the Iran drama, we arrived at a critical turning point in Iraq.

The United States Monday withdrew its combat troops, with a few exceptions, from Iraqi cities, in accordance with a security agreement with Iraqis signed in November. (All U. S. troops are supposed to be out of Iraq by the end of 2011.)

During my last trip to Iraq, in December, it was hard to imagine that this large-scale shift of men and equipment could be completed in such a short time. However, as Gen. David Petraeus told me in an interview, “The process of removing our combat bases from cities is on track and will meet the deadline.” Impressive.

Clearly, many Iraqi players will try to use the pullout to advance their own agendas. Prime Minister Nouri al-Maliki, with an eye toward the January 2010 elections, is calling it a “great victory” against occupation. Others are hoping to restart civil war: A wave of lethal bomb blasts in Shiite neighborhoods last week killed about 200 people, and a car bomb exploded in the northern city of Kirkuk on Tuesday, killing at least 27 people. (Violence overall is still way down, but these incidences stoke fearful memories.)

So the big question is, what happens now? In my phone conversations with several Iraqis, each said the pullback would spur new violence. Many shadowy groups — from al-Qaida fragments to Iranian agents to mafiosi — will test the ability of the Iraqis to take on more of the security burden.

“We have been concerned by the sensational attacks carried out by those trying to revive sectarian violence,” Petraeus said.

The good news is that spurts of violence are unlikely to rekindle full-scale sectarian warfare. Sunnis have been chastened, and Shiites no longer fear the old Sunni order will be restored. “We’ve had a bellyful of that bloody fighting,” one Sunni resident of Baghdad told me, “and no one wants to return to that.”

Moreover, some U. S. troops (the number hasn’t been made public) are staying in cities as trainers and advisers. “There is an agreement reached for the establishment of coordination centers that will remain in Baghdad, Mosul and Basra,” Petraeus said, “and will allow U. S. forces to share information, coordinate activities, and, if required, provide U. S. enablers to assist Iraqi forces.”

Teams of advisers partnered with Iraqi units also would remain, Petraeus added, to ensure sharing of intelligence, coordination of operations outside cities, and access to requested assistance.

If you think this means little has changed, think again. The change is dramatic. The closure of urban U. S. bases and outposts is affecting security in cities, say Iraqi journalists and sources in Baghdad, Fallujah and elsewhere. Iraqi army troops and police don’t go out on patrol as U. S. forces did, which means they have less contact with the population and get fewer tips about bad guys. This makes it easier for criminals to operate, along with other disruptive forces.

In mixed areas, Sunnis still fear the Shiite-dominated security forces are biased. Even in Sunni Anbar province, where the police are all Sunni and violence has plummeted, the U. S. pullback has been accompanied by an uptick in roadside bombs, maiming locals. The perpetrators may want to convince Iraqis that their current system cannot work.

A source in Fallujah told me that, even though most locals wanted the Americans to go, they are nervous. Al-Qaida elements still lurk, and the police are corrupt and hapless. “People don’t know whom to follow,” the source told me.

U. S. officials are hoping Iraq’s political leaders will tamp down renewed sectarian violence or any flare-ups between Kurds and Arabs in the north. Much will depend on the performance of Prime Minister Maliki, who built support by taking credit for the sharp decline in violence since 2006 and 2007.