By Andrew North
BBC Baghdad correspondent, Sadr City
Lt Col Chad Mcree of the US army is on a charm offensive in Sadr City, out and about with his soldiers talking to people on the streets.
Col Mcree is appealing to Iraqis to give the Americans more time
"We want some law and order," says a man at a stall selling shoes.
"We'll do our best but we need your help to do it," Col Mcree tells him.
It is the early days of a new US and Iraqi operation aimed at restoring government control over this vast and rundown Shia district of north-east Baghdad, the latest stage in the new security plan for the capital.
Home to perhaps two million people, a third of Baghdad's population, it has long been under the sway of the controversial Mehdi army militia of the firebrand Shia cleric Moqtadr Sadr.
I've come into Sadr City with the Americans. When I last came here three months ago it was with the Medhi Army's say so and protection.
But now the black-clad fighters who watched my every move then seem to have melted away. Many have reportedly left Baghdad or crossed into neighbouring Iran.
What no one knows yet is whether they have gone for good.
Friendly but bemused
"I know the traffic is bad," Col Mcree apologises to an elderly gentleman in a white and black checkered kefiyeh or head-dress.
He doesn't make any promises to fix that. Part of the reason for the traffic problems is the line of heavily armoured US humvees blocking the road.
Sadr City was until recently a no-go area for the Americans
For the most part the soldiers, who are from a military police unit, get a friendly reception. But there is surprise and bemusement on many faces too. People are not used to seeing US soldiers walking amongst them, especially here.
Until recently, this was a virtual no-go area for the Americans. And although the colonel shakes hands enthusiastically with everyone in his path, some of his soldiers look nervous.
The push into Sadr City has begun with hundreds of US and Iraqi troops moving through the low-built houses and rutted lanes on its western side.
This is the largest American presence here in almost three years.
This is no full frontal assault though. The arrival of US forces was negotiated with local leaders and the Iraqi government. And Mehdi Army fighters who are still there have been told not to confront the Americans.
But like Col Mcree and his military police, other American units are trying to tread carefully.
US vehicles have been rolling through Sadr City as part of the push
In effect, this is a credibility test - for the Mehdi Army as well as the Americans and their Iraqi government allies. It is about who can be best trusted to provide for the people here.
The militia won support in Sadr City because they provided some protection against the constant onslaught from Sunni insurgents. It became a state within a state. The police may have had official uniforms but they answered to the militia.
Like another Shia organisation, the Hezbollah in Lebanon, the Mehdi army has also become a kind of local government - providing cooking gas and services such as fixing drains.
The links go further. Although it is the stern face of Moqtadr Sadr who is the most common sight in Sadr City, you will also see many posters of Hassan Nasrallah, the Hezbollah leader.
But the Mehdi Army has also been blamed for fuelling much of the sectarian violence over the past year, killing hundreds of Sunnis its death squads have rounded up across Baghdad.
The Americans have wanted to go in harder against these death squads before, but have been held back by the government of Prime Minister Nouri Maliki, who is dependent on Mr Sadr politically.
But many believe Mr Sadr is now much weaker.
Pressure on both sides
Ending Baghdad's communal strife is the main goal of the security plan.
Ultimately it is the performance of the Iraqi authorities that will decide whether it works here, whatever the Americans do.
Col Mcree and his MPs were hoping Iraqi police from a nearby station would come out with them on the streets.
But the police didn't turn up - probably concerned at the risk of being seen with US troops. As elsewhere, the translators working with the Americans keep their faces covered in case they are identified.
A young man in the crowd joins in, shouting out to the colonel: "It's safe here in the city (meaning Sadr City), except for the car bombers. That's the real problem."
When he says car bombers, everyone knows who he is blaming - Sunni insurgents, who are believed to be responsible for most of the big attacks. They are the other side in the sectarian war.
"Well help us get rid of them," says Col Mcree optimistically. "You gotta tell us so we can get rid of them."
Not far away was the site of a devastating multiple car bomb attack last November, which killed more than 200 people - the most devastating series of attacks in a single day since the 2003 invasion.
Things are a critical point right now. The Mehdi Army is under real pressure
But insurgent attacks on Shias again appear to be on the rise again, notably with a series of bombings of pilgrims on their way to Karbala, south of Baghdad, for the annual Arbaeen ceremony marking the end of 40 days of mourning for Prophet Muhammad's grandson.
In the latest attack, at least 90 Shia pilgrims were killed by a twin suicide attack in the town of Hilla.
Some say this is happening because the Mehdi army is not there to protect people any more.
I go to see the police chief of Sadr City, Col Abdul Zahra Hameed.
"People are still scared," he says. "We hope the arrival of the Iraqi and American forces will break this barrier of fear."
But there is a long way to go. American and Iraqi forces may be back in Sadr City but they've still got a lot to do to gain the people's trust.