The war in Iraq has been one of the most disastrous wars ever fought by Britain. It has been small but we achieved nothing. It will stand with Crimea and the Boer War as conflicts which could have been avoided and were demonstrations of incompetence from start to finish.

The British failure in the Iraq war has been even more gross because it has not ended with a costly military victory but a humiliating scuttle. The victors in Basra and southern Iraq have been the local Shia militias masquerading as government security forces.

Britain should immediately hold a full inquiry into the mistakes made before and during the war in Iraq out of pure self-interest. Gordon Brown's suggestion that holding such an inquiry now would somehow threaten the stability of Iraq is either a piece of obvious prevarication or, if taken at face value, a sign of absurd vanity. Iraqis show not the slightest interest in British policy and assume it will simply be an echo of decisions made in Washington.

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I have watched this war being fought over the last five years and I never for a moment felt that the Government in London had the slightest idea of the type of conflict in which it was engaged. It has become common for supporters and opponents of the war to argue patronisingly that what was needed was a plan about what to do after the war, as if this would have reconciled Iraqis to be occupied by foreign powers.

Those British officers I met over the years had an acute idea of why intervention in Iraq was a very bad idea but had become used to being ignored. A few would claim that Britain had rich experience of counter- insurgency in Malaya in the 1950s and Northern Ireland after 1968. "The situation in Basra was exactly the opposite," one former British military intelligence officer exclaimed to me impatiently. "In Malaya and Northern Ireland, we had the support of the majority but in Basra we have no allies."

How we got into this situation needs to be inquired into and also how we avoid falling into it again. The worst failings were political. In many ways Tony Blair in 2002-03, when he decided to join America in the war, resembled Neville Chamberlain in 1938. He ignored expert professional advice. He had no alternative plan if anything went wrong. He lived in a world of propaganda and fantasy. He would spring from his plane in Baghdad to be greeted by Iraqi politicians who did not dare leave the Green Zone.

There are 175 British servicemen who have died for nothing. The troops stationed outside Basra do nothing except show the US that they have one ally left.

The British Government throughout the whole war has shown an extraordinary degree of arrogance and ignorance of history. They did not seem to know that three years after Britain captured Baghdad in 1917 it was fighting a ferocious tribal revolt along the valley of the Euphrates.

It does not require much knowledge to understand that any country should be chary of being sucked into small wars. The Duke of Wellington, who had seen what had happened to Napoleon in Spain, said that "Great powers do not have small wars". Most of the reasons why Britain should not have allowed itself to become the unquestioning ally of America in what became an imperial occupation are obvious.

America and Britain discovered Iraq was a quagmire still. If the military situation has stabilised it is only because Iraqi Sunni and Shia now hate each other more than they hate the Americans. It is a terrible legacy of five years of war.

*Senator John McCain arrived in Baghdad yesterday for an unexpected visit to Iraqi and US diplomatic and military officials.

Details of the visit by one of the foremost supporters of the 2003 invasion and soon-to-be Republican presidential nominee were not being released for security reasons, the US embassy in Baghdad said.

"Senator McCain is in Iraq and will be meeting with Iraqi and US officials," said an embassy spokeswoman Mirembe Nantongo.

There were no media opportunities or news conferences planned for the visit. Senator McCain, who is believed to be staying in Iraq for about 24 hours, is on his eighth trip to the country.