The 23-year-old absconder described it as “a career move”. The pay was better, the duties less onerous and there was far less chance of being killed.

Three years after President Bush declared “mission accomplished” in Iraq, young gunslingers such as Alah are what passes for the law across much of this city today.

Nobody knows for sure the strength of Iraq’s militias, but they certainly outnumber the 120,000-strong police force that estimates it is losing several hundred recruits a month. This is the only country where police and soldiers have it written into their contracts that they can leave on a whim without being punished.

Alah’s defection is a blow to attempts to rebuild Iraq. Western money and manpower trained him to replace the British and US forces. But, with young recruits deserting in ever growing numbers, the prospect of a swift pullout recedes still further.

Nouri al-Maliki, the new Iraqi Prime Minister, has sworn to put Alah and his kind out of business. Although most of Mr al-Maliki’s countrymen applaud that idea, they are intrigued to see how he can disband the militias without provoking even more bloodletting.

Dressed in the black uniform of the Mahdi Army, Alah told The Times that with his father dead, and as the oldest of ten children, it was his responsibility to be the main wage earner.

After years working on building sites and a stint as a conscript in Saddam’s army, Alah was out of work when he was picked to join the elite ranks of the police rapid reaction force, dealing with riots and terrorist attacks. He knew the risks. No police force on Earth has suffered more casualties this year, but he felt that the £140-a-month salary was worth it. But al-Mahdi pays him a lot more.

Alah was brought up in a Shia neighbourhood but laughs at the idea that it was religious conviction that encouraged him to join the ranks of the Mahdi Army. “It is an attractive package,” he says, weighing up the economic advantages offered by the militia, such as a pledge to take care of Alah’s family if anything happens to him. He and his colleagues do as they please. They do not bother with warrants before searching premises, and can open fire at will.

As well as dispensing rough justice, al-Mahdi is behind the increasingly oppressive campaign against “antisocial behaviour”, which means harassing young women in the street. Yet in its heartland of Sadr City, a teeming slum northeast of the capital, there is no denying that Alah and the dozen other young men in his platoon are respected and feared by local families who claim to have lost faith in the police.

However, it is often hard to distinguish between the two as the militias regularly wear identical blue uniforms, carry the same weapons and drive blue- and-white marked squad cars.

Alah refuses to say whether he has been responsible for the death of anyone in uniform, or taken part in attacks in which civilians have been killed. More than 160 Iraqis have died in violent attacks during the past week, most of them perpe- trated by militias.

Alah and his colleagues roar around Baghdad in a fleet of powerful US-manufactured vehicles, sirens wailing and guns pointing out of the windows as they carve through the city’s choking traffic jams.

At one stage, two of his team navigated a roundabout in opposite directions in a race to be the first to reach one of the city’s main dual carriageways. At the tail of this cavalcade was a Toyota pick-up truck with four men, their faces hidden by black balaclavas, hanging on to a heavy machinegun.

The police melt away at the sight of them, recognising that they would come off second-best in a skirmish. To show their contempt for US forces, these unlicensed private armies race around close to the fortified green zone, where the international community and the Prime Minister-elect are in residence.

Mr al-Maliki recognises the scale of the problem that he faces in amalgamating the militias into the country’s security forces when 11 of his main political rivals in parliament run their own armies.

He was supported by the country’s most revered cleric, Grand Ayatollah Ali al-Sistani, who said that only government forces should carry guns. But hours later Mr al-Sadr contested this, saying that the Shia population felt safe only when his army was on the streets.

Alah makes it clear that he has no intention of going back to his old job. “I much prefer this life,” he says, patting the police-issue Glock pistol hanging from his hip.

• President Talabani has held talks with insurgent groups to secure a ceasefire after at least two dozen people died during a weekend of bombings and drive-by shootings. The President said that he was optimistic that a truce could be agreed.

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