Three quarters of patrols here result in clashes with insurgents, so trouble is likely. It is just a question of when and where. “Hey,” Sergeant Walton, a US team leader shouts to his interpreter. “Make sure that when the shooting starts the Iraqi commander knows only to let his men nearest it start firing back.”

The Americans are technically on a familiarisation mission, guiding the 303rd Iraqi Army Battalion through the streets as a prelude to handing the zone over to them next week. But they have good reason to call for caution.

I do not know what effect the Iraqi soldiers will have on the enemy, but they terrify me. An eagerness to pull the trigger gleams in their eyes as they wave their Kalashnikovs about.

They have a reputation for spraying bullets all around them if fired on, and two Americans have been killed by such stray rounds. “I’m more scared of going out with these guys than clashing with the insurgents,” an American trooper says. “They have no concept of identifying friendlies, and let loose at anything.”

Haifa Street: its name has become synonymous with all that has gone wrong in Iraq. Flanked by high-rise apartment blocks, it was once a wealthy thoroughfare in central Baghdad, home to Iraqi and foreign Baathists, hardcore loyalists favoured by Saddam. Behind these apartments lie the ghettos through which the US and Iraqi troops now move, alleys that were always lawless .

Since the war the worst elements of each community — Syrian and Iraqi Baathists, ghetto mafia and Islamic fundamentalists — have formed a daunting alliance that has transformed the zone into an urban battlefield.

Young children are used by the insurgents to lob grenades from rooftops on to coalition troops below. Snipers and men with rocket-propelled grenades conduct frequent ambushes.

Sergeant Walton’s platoon has escaped lightly: only 20 of its 36 men have been wounded. In other platoons of Charlie Company, 1st Battalion, 9th Cavalry Regiment only a handful of troopers remain unscathed. “Stay close,” Sergeant Walton tells me. It is all I can do not to clutch his belt.

There is the occasional crump of a grenade and chatter of gunfire. Yellow smoke rises from a nearby square, suggesting that US advisers to a neighbouring Iraqi battalion want a casualty evacuation.

Then seven mortar rounds crash on the troops nearing al-Jabouri mosque, knocking down and injuring two Iraqi soldiers and an American. The mosque’s imam, an anti-coalition firebrand disliked by the Americans, is also wounded by shrapnel, taking an undignified wound in his bottom. He allows the soldiers to patch him up, then starts cursing them.

The Iraqi forces appear excitable but unfazed by the fighting around them. The general impression is that they will be unable to cope with the insurgency. That may be wrong; their courage cannot be underestimated.

Before the patrol started I watched them queue goodhumouredly to eat in the cookhouse of the nearby Forward Operations Base — each man was searched before he could enter. Two hours earlier a suicide bomber had killed more than 20 potential recruits at the base’s southern gate.

When the area was cleared up, more recruits arrived. More troops have been murdered by insurgents in their homes than killed on operations, yet still they join up.

At least 75 per cent of them are Shia. In some units it is 90 per cent. How they will be received by Sunni communities is unclear. “The Shia love them,” said Captain Eric Massey, a company commander with the 82nd Airborne whose troops are attached to the Haifa Street operation. “The Sunnis here are either neutral or less excited.”

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