Lina Massufi, a 32-year-old Iraqi laboratory assistant with two children, is a widow - her husband was killed by US troops when he accidentally drove down a closed road in 2003. In the past three months she has seen her house raided and her furniture smashed 12 times.

"Every time they raid my house, they break down the door," she told a UN official. When she asked them why they did not ring the bell "they laughed at me and called me an idiot". Her brother Fae'ek, a pharmacy student, was arrested and held in prison for a week. "He returned with signs of torture on his body, and was crying like a baby because of the pain."

Her story shows why the odds are against what may be President George Bush's final gamble in Iraq: the attempt by US troops, now receiving 17,500 reinforcements, to regain control of Baghdad. The plan is for US forces, along with Iraqi army and police, to enter Sunni and Shia districts in the capital, cleanse them of insurgents and militia and then stay put, preventing their return. In his State of the Union speech last week Mr Bush told Congress: "With Iraqis in the lead, our forces will help secure the city by chasing down the terrorists, insurgents, and the roaming death squads."

But the failings of this strategy become more obvious the further one gets from Washington and the closer to Baghdad. The insurgents and militiamen, both Sunni and Shia, usually have more credibility in their districts than Iraqi government forces. As for the heavily Shia police commandos, they are seen by Sunni in Baghdad as licensed death squads.

A foretaste of what the "surge" of US and Iraqi soldiers will mean came last week, as they fought their way into the tough Sunni insurgent-controlled Haifa Street neighbourhood, only a mile from the Green Zone. Iraqi soldiers happily let US forces take the lead, and a US long-range missile demolished a house from which snipers were allegedly firing. The readiness of the Americans to use such heavy weapons in densely-populated urban areas ensures that many civilians have been, and will be, killed and wounded.

The Iraqi government forces are either highly sectarian or will not fight. The insurgents and militias are strong because they provide the security the government does not, and Baghdad has already broken up into several dozen hostile townships, each defended by its own militia. There are fewer and fewer mixed districts; Shia caught in Sunni areas are killed, and vice versa. Strangers are viewed with suspicion, and there are signs everywhere, saying "Death to Spies".

The American troops may be seen as temporary allies by either side, but are also blamed for the lethal anarchy. Some 61 per cent of Iraqis, a majority of both Sunni and Shia, approve of armed attacks on US forces.

The Shia, the majority in Baghdad, are on the offensive. They have their great bastion in the shabby overcrowded houses of Sadr City, with more than two million people, and have taken over almost all of Baghdad east of the Tigris, aside from a few hard-core Sunni areas such as al-Adhamiyah. They are also seizing ground in west Baghdad, attacking south from al-Khadamiyah, site of a revered Shia shrine. Al-Hurriya, once mixed, is now Shia; the Sunni are being pressed back into the south-west of the city.

In the heart of this Sunni core of Baghdad, now under insurgent control, lies al-Khadra. The sort of area where the future of the US plan will be decided, it used to be a modestly prosperous 1970s suburb, bisected by important highways now leading to the US military headquarters at the airport, the half-ruined city of Fallujah and the notorious prison at Abu Ghraib. Another highway leads to Taji, north of Baghdad, where there have been repeated insurgent attacks.

Al-Khadra's 60,000 people are waiting with dread to see what the coming US-Iraqi government offensive means for them. There is a lot for them to be frightened of: already young men of military age are leaving the neighbourhood for Syria or Mosul in northern Iraq.

Ismail, in his early thirties, fled to Mosul when police commandos suddenly arrested 25 men, including two of his cousins, on 15 January. One cousin was later released and told how he had been beaten and tortured with electricity, accused of being an insurgent. "When I said I was not a mujahedin [fighter], they said that as a Sunni I was bound to support them." He was only released when a US commander demanded to know what had happened to the prisoners, and the commandos freed 15 at random. The fate of the others is unknown.

I first visited al-Khadra in October 2003, when a police station near a mosque with a green dome had been attacked by a suicide bomber, killing three policemen and wounding nine. I walked over broken rubble to talk to shaken policemen who survived. They were barricading off the street in front of their station, and it has remained closed to this day. But while the barricades may stop suicide bombers, insurgents have repeatedly attacked with mortars and rocket-propelled grenades (RPGs).

Al-Khadra was predisposed to favour armed resistance to the US. Many of its people had worked for the old regime - under Saddam Hussein, when the district was 80 per cent Sunni and 20 per cent Shia, several senior leaders of the Baath party lived in al-Khadra. Sunni inhabitants often came from the great tribes of western Iraq, such as the al-Dulaimi, al-Rawi and al-Hadithi, who are at the heart of the insurgency.

US vehicles often used the important roads leading through al-Khadra, and as often came under attack. The most common weapon was the roadside mine (the notorious IED, or improvised explosive device), frequently consisting of several heavy artillery shells wired together. This was planted either in the road or among the piles of rubbish that lie all over al-Khadra. The insurgents, realising the usefulness of garbage for concealing mines, discouraged rubbish collectors by simply shooting two of them dead.

The men who detonated the mines became expert at timing them to explode when US patrols were passing. Once I went to a house on the edge of al-Khadra, facing a highway on which a US Humvee had just been blown up. The heavy machine gunner had been killed and his gun, its barrel twisted sideways by the blast, had been hurled 40 yards on to the roof of the house.

In February 2006, when the Shia al-Askari shrine in Samarra was blown up, there was a pogrom of Sunni in Baghdad. Many survivors moved into al-Khadra, and at the same time the Shia were driven out. "Some were threatened; many just fled," said a resident. "Now, if a Shia is found here, he is killed." The insurgents who took over were preferred to Iraqi government forces, and deemed essential in case of an attack by Shia militias like the Mehdi Army.

How would the people of al-Khadra react if US troops and Iraqi security forces launched an offensive? Probably some of the insurgents would fight to the last, but others would fade away, using classic guerrilla tactics. "Searches we could accept, and maybe the presence of Americans and Iraqi army," said Ismail, "but not mass arrests or the use of the police commandos. If this happens, we will resist."

THE FLASHPOINTS

Sadr City

Stronghold of radical Shia cleric Moqtada al-Sadr's Mahdi army. Fighters can move from rooftop to rooftop

Al-Adhamiyah

Traditional Sunni district, vulnerable because it is on the Tigris's east bank, now overwhelmingly Shia

Al-Khadimiya

Site of one of the holiest Shia shrines, now infiltrated by Shia militias pushing into western Baghdad

Karada

Shia area more peaceful than most, but increasingly targeted recently by bombs and suicide attacks

Intense streetfighting last week in the Sunni insurgent-held district of Haifa Street, when US forces used heavy weapons in a densely-populated area, raised fears that the struggle to regain control of Baghdad could resemble the battle of Stalingrad, with residents of districts such as al-Khadra vowing to resist.