As the US begins to acknowledge the magnitude of its defeat in Iraq, the conflict looks more than ever like a speeded-up, scaled-down re-enactment of Vietnam. A tragedy that took a dozen years to unfold in south-east Asia has played out in less than four in Mesopotamia. Once again an intervention that sprang largely from idealistic, anti-totalitarian motives has gone awry because of an administration’s deceptions, incomprehension and incompetence. Once again the domino theory at the heart of the case has been disproved – and once again America finds itself looking for a way out that will not compound the catastrophe.

As in the final stages of the Vietnam war, the US faces the question: if it has lost, why is it still there? One answer is that President George W. Bush is a stubborn man. Even this week, Mr Bush was insisting it would not withdraw “until the mission is complete” – an apparent synonym for “when hell freezes over”. A better answer is that the US is now in Iraq to prevent genocide. Without a military force separating Sunni and Shia, the present savagery could turn Cambodian, with remaining secular democrats as the first victims. A power vacuum could provide a new operational base for al-Qaeda and severe sectarian violence could spiral into all-out civil war and regional conflict. As awful as it is now, Iraq could get much worse.