Rupert Cornwell: Thanksgiving Day massacre highlights waning US power

It may or may not go down in Iraq's dreadful recent history as the "Thanksgiving Day massacre". But one thing is certain: the car-bombing that killed more than 130 people in the Sadr City district of Baghdad could not have come at a worse moment for the beleaguered Bush administration as it seeks to end the sectarian violence tearing the country apart and get US troops home.

Like the killing of Pierre Gemayel in Lebanon on Monday, the atrocity might have been timed to expose how the US - its military power and moral authority sapped by the war - has lost most of whatever influence it had to shape events in the Middle East.

Over the next few days, President George Bush and Vice-President Dick Cheney will lead, in person, an unparalleled diplomatic effort by Washington to find a face-saving exit formula. It comes ahead of the completion of keenly awaited Iraq policy reviews by the Pentagon and by a bipartisan commission headed by the former secretary of state, James Baker.

Mr Cheney leaves today for Saudi Arabia and talks with King Abdullah. At the top of the agenda will be a possible role for the Saudis in reining in the Sunni insurgency, suspected of being behind the bombings in Shia Sadr City. Early yesterday, Iraqi media said the Vice-President had brought forward his trip in order to make a surprise Thanksgiving Day visit to the 145,000 US troops in Iraq, similar to the secret journey made by Mr Bush in November 2003. However, the White House swiftly denied the report.

Next week, the President himself goes to the region to confer with Nouri al-Maliki, the Iraqi Prime Minister. But, in a sign of how dangerous Baghdad has become, the meeting will take place on the far safer territory of Amman, Jordan.

Officially, it is billed as a show of confidence by Washington in the embattled Prime Minister, who has been strongly criticised by administration officials for his failure to take on the Shia militias who control parts of the country. That will again be a key topic at the Amman talks, along with the need for measures to restore order in Baghdad itself - accepted as the prerequisite for any settlement that would permit US forces to leave.

But the triple car-bombing yesterday, carried out almost certainly by Sunnis, has shown yet again how hard that task will be. US commanders have acknowledged that a joint crackdown in Baghdad by American and Iraqi forces in the summer has not succeeded. Its failure was underlined by United Nations figures showing that 3,709 Iraqi civilians were killed by the violence in October, the highest monthly figure yet.

Now the Pentagon and the Baker commission are deciding whether to sent an extra 20,000 or 30,000 US troops to Iraq, and, in particular, to Baghdad, in a last-ditch effort to get a grip on the violence. But yesterday's events suggest even that may not be enough.