A one-armed man who lives in my constituency claims that American troops pepper-sprayed him in the face causing him to vomit, flushed his head in a toilet pan and then, having tied him up, knelt on him and punched and kicked him. He says that he endured many brutalities during nearly two years at Guantanamo Bay.

When I first read his allegations I considered them absurd and I thought the newspaper that printed them gullible. A US military spokesman called them “simply false”. Following the scandal of prisoner abuse in Abu Ghraib jail in Baghdad, of course I now believe him. His testimony is very consistent with what we have seen in photographs and videos.

His reliability as a witness is damaged because he has a mild criminal record and he was arrested in Afghanistan during the war against the Taliban. That I now accept his word rather than the statement of an American official is a symptom of how much credibility the United States has lost, even among ardent admirers like me.

I do not repent of having supported the war in Iraq. Saddam Hussein’s defiance of international law posed a danger to the region and as his scientists worked on weapons of mass destruction the risks could soon have spread wider. American weakness in confronting both him and, separately, Al-Qaeda between 1991 and 2001 increased the West’s vulnerability. Particularly after withdrawing from Somalia, following the loss of a few men in 1993, the United States looked timid. Its responses to terrorist outrages and Saddam’s provocations were half-baked. The younger Bush’s robust foreign policy was a welcome change from Clinton’s.

If Bush had other motives, too, so much the better. People who like conspiracy theories say the war in Iraq was really about oil. Well, if America is anxious to secure the energy supplies that make life possible in the modern world, that is not an unworthy aim. In fact, America was not especially interested in Iraq’s oil because we can just about do without it.

It was concerned to have troops in the Middle East who could move to protect oilfields and pipelines elsewhere. But keeping forces in Saudi Arabia, the land of the holy places, was proving offensive to Muslim sensitivities. We Europeans, who showed little gratitude to America for decades of protection against the Soviet Union, have also shown Olympian disdain for what is in effect an American investment in keeping our schools and hospitals heated and lit.

The neo-conservatives who came to office with George W Bush are experienced advisers who form a sophisticated cabinet. Some are my friends. I have a lot of sympathy with their views. For example: their lack of enthusiasm for a United Nations bedevilled by corruption. Bush, though inarticulate, may be a second Ronald Reagan, able to set clear and simple policy objectives. But it is astonishing that such a formidable executive has made so many disastrous mistakes.

After September 11 the script for the war on terror wrote itself. Not since the days of Adolf Hitler had allies been so certain of being in the right, battling against an unspeakable evil. Brutal terrorists born and bred in the darkness of repressive regimes threatened our freedom and plotted to smash the value system that brought the world justice and prosperity.

It beggars belief that the US government did not see from the outset that its conduct in the wars against the Taliban and Saddam had to be beyond reproach and that if it were not, the whole moral basis of the West’s campaign would collapse.

I do not often agree with our archbishops, but their letter to Tony Blair (it came to light last week) is spot on. Referring to the prison atrocities it said “the appearance of double standards inevitably diminishes the credibility of western governments with the people of Iraq and of the Islamic world”.

Winning the support of Muslims was bound to be hard. An Arab street which thinks that the CIA and Mossad flew the airliners into the twin towers would believe anything, but the photographs from Abu Ghraib have humiliated the democracies. We have been left speechless in the face of atrocities committed by other regimes.

The British government’s indignation at the humiliation of our servicemen by Iran rings hollow. During his visit to Britain the Chinese prime minister could hardly contain his mirth when the subject of human rights abuses was raised at a joint press conference with Blair.

In part my indignation against the American administration arises from sympathy for our prime minister. No leader of this country would by a nod or a wink condone the mistreatment of detainees. I empathised with Blair during the war as he strode forward in a glow of idealism. Proudly Britain stood shoulder to shoulder with the land of the free against the tyrant. Guantanamo and Abu Ghraib have mocked us all. Nonetheless Blair has failed to answer satisfactorily about when he first knew of the atrocities and when, if ever, he protested to Bush about them. When this issue was hot a few weeks ago I believe I saw a look of terror in the prime minister’s eyes. Those who want to bring him down should probe further.