When he had finished photographing the bodies, he gingerly carried them out of the house. Six of the slain were children. He picked up the corpse of a little girl.

“I held her out like this,” Briones said, spreading his arms, “but her head was bobbing up and down and the insides fell on my legs.”

The shocked 21-year-old was neither a survivor, nor a relative. He was a member of the “elite few”, as the US marines style themselves. Earlier that day, his comrades in arms from the Kilo company 3rd battalion, 1st marine division, had gone on a killing spree. Briones, the clean-up guy, was sickened. It “left something in my head and heart”, he said.

For Donald Rumsfeld, the American defence secretary, this horrific incident shows that “things that shouldn’t happen do happen in war”. Consciously or not, it is an echo of his brutally dismissive “stuff happens” when looting broke out in Baghdad after the toppling of Saddam Hussein.

Three years on from the invasion, however, Rumsfeld can no longer shrug off such a disaster. Already tainted as torturers by the prisoner abuse scandal at Abu Ghraib, the heroes and liberators of Iraq now face accusations of murder. President George W Bush promised last week that the results of an official inquiry will be published.

The danger for the beleaguered president and the US military is that the execution-style killings at Haditha could mark a turning point in America’s perception of the war.

Old soldiers still remember the merciless taunts of “baby-killers” which dogged them after the Vietnam war. The horror at My Lai, where up to 500 Vietnamese villagers were mown down by US troops in 1968, turned a war-weary public against a conflict that was corroding American values. Could the alleged massacre of 24 Iraqis at Haditha have the same impact?

In scale the two tragedies are barely comparable, but the American military was supposed to have learnt its lesson. Most of the My Lai soldiers had been drafted in and were scared and inexperienced.

Today the army is a professional, all-volunteer force. Marines are trained to kill, but only under orders in a disciplined fashion. At Haditha, a desert town riddled with insurgents on the edge of the Euphrates, they cracked.

An American government official has admitted the marines “suffered a total breakdown in morality and leadership”. Disturbingly, the incident may not be an isolated one.

The new Iraqi prime minister, Nouri al-Maliki, said angrily last week that US forces were killing civilians “just on a suspicion or a hunch”.

Hours later, allegations surfaced of another massacre, this time of 11 Iraqis, including four children and a six-month old baby in the town of Ishaqi in March. A video obtained by BBC television appeared to show that some of the victims had been shot in the head.

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