Realisation that the US faces defeat in Iraq is sinking in even among the war's greatest supporters.

Senator John Warner, the chairman of the Armed Services Committee, may only have said after a visit to Baghdad last week that "the situation is drifting sideways" and that "a change of course" would be necessary unless violence diminishes. But these are code words for a US withdrawal.

The US media are quick to sense which way the political wind is blowing. Suddenly it is full of negative stories that American reporters, however well-informed and intrepid, had difficulty getting on the air a couple of years ago. It is also becoming clear that the much-heralded US offensive to regain control of Baghdad has failed, amid a rise in the American death toll. Some 242 US soldiers have been killed and wounded in the first week of October alone.

It is extraordinary how little control the US and the Iraqi government exercise over Iraq, three and a half years after the overthrow of Saddam Hussein. Last month I was in Mosul, the northern capital of Iraq, where American forces have retreated to a single base at the airport outside the city. In eastern Iraq I was in Diyala province, where again there is an American base outside the provincial capital of Baquba, but local Iraqi police commanders say the Americans seldom venture out.

American politicians are suddenly asking the questions that they should have asked long ago. "We've heard over and over again that as Iraqis stand up our troops will stand down," said Republican Senator Susan Collins, chairman of the Senate Homeland Security Committee. "Well, there are now hundreds of thousands of Iraqi troops and security forces, and yet we have not seen any reduction in violence." Her point is crucial - there are supposedly 250,000 Iraqi troops and police being trained to take over from the Americans and British. But their loyalties are less to the Iraqi government than to their own communities.

As a result, civil war is raging across central Iraq. The government of Nouri al-Maliki has failed just as dismally as that of the previous Prime Minister, Ibrahim al-Jaafari, which the US and Britain took great pains to displace earlier this year. Both became governments of the Green Zone, dependent on the Americans and lacking legitimacy in the eyes of Iraqis.

The US and Britain have never faced up to the extent to which their occupation of Iraq delegitimises any Iraqi government and deepens the divisions leading to civil war. In theory, sovereignty was handed back with great fanfare in June 2004. In practice this never happened. Defence and intelligence remained under US control. The present Iraqi government would not last a day without the presence of 140,000 US troops.

'The Occupation: War and Resistance in Iraq' by Patrick Cockburn is published by Verso tomorrow. 'The Independent on Sunday' will publish an extract next week