Two and a half years later Powell referred to the episode as "a painful blot" on his record - a pack of lies and half truths that has led to an ever-increasing mound of corpses.
The power of both illusion and delusion should never be underestimated. The compulsion to believe in something we need and want to be true, rather than see reality for what it is, can at times be astounding.
Remember those eyewitness accounts of the Brazilian student Jean Charles de Menezes before he was shot dead on the underground? Not all of them were made up by the police, although they did nothing to deny them. Mark Whitby, a plumber from Brixton, thought he saw a Pakistani terrorist being chased and gunned down by plain-clothes policemen. Less than a month later Mr Whitby told the Daily Telegraph "he now believes that what he actually saw was the surveillance officer being thrown out of the way" as Mr Menezes was being killed.
Anthony Larkin, who was on the train, said he saw "this guy who appeared to have a bomb belt and wires coming out". The Pakistani in the puffa jacket who vaulted the barriers, it transpired, was a Brazilian in a light denim jacket who picked up a free paper and used his Oyster card.
I am not talking here about lying. The potency of downright fabrication is self-evident. What is truly insidious is the propensity of people to arrange an array of possibles, probables, maybes and might-bes, and construct from them a reality that is both definite and wrong.
The power of suggestion, assumption and presumption is everything. The day before Menezes was shot, London saw an attempt to launch a second terrorist attack in two weeks. What Whitby and Larkin saw had been refracted through a prism of fear and stereotypes, and emerged completely distorted. The price was right; the market was ripe; people bought into it.
The war in Iraq has revealed just how truly bullish and persistent this market in bad ideas based on flawed preconceptions can be. Bad ideas helped take us into the war; and unless we examine what they were and why some managed to believe them, they will prevent us from getting out.
In such a market there will always be sellers aplenty. Someone, somewhere, will forever be peddling war, bigotry, conspiracy, profiling, persecution and plunder. It is only when the buyers come forward in large numbers that we really have to worry. For at critical moments people do not just consume these bad ideas; they invest heavily in them too. So when reality refuses to match up to the idea, they do not change their ideas; they change reality.
There were of course lies; huge whoppers served up on both sides of the Atlantic. On February 23 2003 Tony Blair told the Commons that the government was giving Saddam "one further final chance to disarm voluntarily" through the United Nations. Three weeks earlier President George Bush told him the war was going ahead regardless of what the UN decided. Blair replied that he was "solidly" behind him.
This is of course disgraceful, not least because those who lied have never accepted responsibility for their actions. But it was not a surprise. The case was always flimsy and those who made it were never trustworthy. What is shocking is the number of people who not only bought it but wore it and are still trying to sell it on.
Last October the former Democratic presidential hopeful John Kerry said: "I regret that we were not given the truth; as I said more than a year ago, knowing what we know now, I would not have gone to war in Iraq. And knowing now the full measure of the Bush administration's duplicity and incompetence, I doubt there are many members of Congress who would give them the authority they have abused so badly. I know I would not."
If Kerry did not have the full measure of Bush's duplicitous and incompetent nature by that stage then he is a poor judge of character. The overwhelming majority of people in the rest of the world - who had far less access to information than he did - managed to see the war for what it was. But then they weren't going to run for president.
In November the former Powell aide Colonel Lawrence Wilkerson told the Today programme: "You begin to speculate, you begin to wonder ... was this intelligence spun? Was it politicised? Was it cherry-picked? Did, in fact, the American people get fooled? I'm beginning to have my concerns." Go figure. Shouldn't the speculation begin before the bombs drop rather than after?
Everybody has the right to change their mind and make mistakes. The growing number of people on both sides of the Atlantic who believe it was wrong to go to war is heartening. But since the war has already been going for almost three years these regrets are only of any use, beyond personal expiation, if they help to correct the consequences of the original sin.
These particular turnarounds fail on two fronts. First, they expose the anti-war case to the charge of opportunism. People such as Kerry backed the war not on principle but because it was expedient to do so. They oppose it today for the same reason.
Second, there is little point in claiming you were tricked unless you address what made you so gullible in the first place. The basic idea that the US has a historic duty to bring progress, democracy and enlightenment at the barrel of a gun seems about as firmly ingrained in the American mindset as its record of doing the opposite in Central and South America and south-east Asia is in American history. Nothing that has happened in Iraq seems to have shifted that perception in the US. A significant minority were against the war from the start. For the rest, the trouble with the war is not that they invaded a sovereign country on a false pretext and killed hundreds of thousands. It's that they're not winning.
"We can't leave Iraq. We simply can't," says Colonel Wilkerson. "We're there, we've done it, and we cannot leave." Kerry's position is similar. A Pew research survey in December showed that 48% of Americans believe that invading Iraq was wrong. A Los Angeles Times/Bloomberg poll last week revealed that 57% of Americans support military intervention if Iran builds itself a nuclear capability.
With each exposé of torture, subjugation, blunder and plunder you keep hearing that Americans have lost their innocence. Somehow they always find it again just in time to buy into the next bad idea.
