There should be at least one universal response to this in Britain. Why is Tony Blair still Prime Minister after leading his country into such a disastrous war? Any large company would by now have got rid of a managing director guilty of a mistake on that scale. Any institution you care to name would have done the same. Why is Blair immune from the normal requirements of high office?

Why, instead of being allowed by the cabinet to establish six new policy committees designed to entrench his legacy, has he not been impeached and thrown out of office? Even if his Iraq policy was formed in good faith, the scale of the error surely requires us to ask him and all those concerned with this disaster to leave.

It doesn't matter now whether you were pro-war, strongly opposed to it or somewhere in between, the policy in the Middle East has been an unmitigated failure, an outcome that was built into the earliest planning for the enterprise. People's views four years ago don't count now because Britain is at the heart of a world-changing catastrophe and as far as our interests go, there has not been a single advantage, not even the one of keeping the special relationship alive.

How did we get here? The answer is still not entirely clear. We think we know that Blair manipulated the situation, but we still don't have all the evidence. What is needed is for people to come forward and for the past to be examined more intensively than before.

For instance, it is well worth returning to a memo written by a young diplomat named Matthew Rycroft, which is still significantly undervalued as evidence of the Prime Minister's drive to war and of the innate negligence of American planning for the period after the invasion.

Rycroft is now safely tucked away in Sarajevo as British ambassador to Bosnia. But in the summer of 2002, aged 34, he was Tony Blair's private secretary for foreign affairs. In this capacity, he attended a secret meeting at Downing Street which included Tony Blair, Foreign Secretary Jack Straw, Defence Secretary Geoff Hoon, Sir Richard Dearlove, the head of MI6, John Scarlett, the head of the Joint Intelligence Committee, Lord Goldsmith, the Attorney General, and Blair's military chiefs and the sofa cabinet - Alastair Campbell, Sally Morgan and Jonathan Powell. He then wrote a memo to his boss, Sir David Manning, Blair's chief foreign policy adviser.

It is really a minute of the meeting. The crucial passage reads: 'C [Sir Richard Dearlove, head of MI6] reported on his recent talks in Washington. There was a perceptible shift in attitude. Military action was now seen as inevitable. Bush wanted to remove Saddam through military action, justified by the conjunction of terrorism and WMD. But the intelligence and facts were being fixed around the policy. The NSC [the US National Security Council] had no patience with the UN route and no enthusiasm for publishing material on the Iraqi regime's record. There was little discussion in Washington of the aftermath after military action.'

The Downing Street Memo, as it became known, was published in the Sunday Times on 1 May 2005, five days before the general election. It certainly made an impact but by the end of that week, it had been washed away with the rest of the pre-election clamour. Blair had won a third term and his mysterious hold over the British electorate managed even to vanquish these revelations about British and American thinking eight months before the war.

It took a while for it to surface in the press in the US although its consequence was immediately grasped in the blogosphere. In Britain, the memo became part of the inconclusive miasma of the Hutton report into David Kelly's death and of the Butler review of intelligence on WMD; and it decomposed in the public's understanding at roughly the same rate. Indeed, one often wonders if Blair has been saved by the amount of material produced by public inquiries (Hutton is 740 pages; Butler 192). The more that is published, the more the issues blur.

But the memo is the goods. It establishes Bush's resolve to find a pretext for war, regardless of the facts on WMD and Saddam's links to terrorism. It further makes plain that there was little or no thinking about the postwar period, an error that now must be regarded as equal to or greater than the invasion. No surprise is expressed in Rycroft's account of the meeting about what was going on in America, which leads one to assume that among a very small group, the idea of invasion was a fully fledged possibility, even though Blair was assuring the public and cabinet colleagues outside the inner circle that nothing had been decided.

There was much more in the original Sunday Times report on the meeting. Jack Straw and Lord Goldsmith had doubts about the legal case for war, while Blair was committed from the outset to supporting US plans for regime change. At the time, no one seems to have remembered what Tony Blair had said in his evidence to Lord Butler's report into the intelligence on WMD, published eight months before the memo came to light. Blair said: 'I remember that during the course of July and August, I was increasingly getting messages saying, "Are you about to go to war?" and I was thinking, "This is ridiculous" and so I remember towards the end of the holiday actually phoning Bush and saying we have got to put this right straight away... we've not decided on military action.'

If not a direct lie, it is hardly the truth.

On the September dossier, Tony Blair said: 'The purpose of the dossier was simply to say, "This is why we think there is intelligence that means that this is not fanciful view on our part."'

It is clear now that he knew the Americans were fixing their intelligence for war and that he had to get his act together. In all the emails that emerged during Lord Hutton's inquiry, the pressure to make this case is clear. Here is one from young Rycroft: 'Part of the answer of "why now?" is that the threat will only get worse if we don't act now - the threat that Saddam will use WMD, but also the threat that Iraq's WMD will somehow get into the hands of terrorists.' Rycroft was helping to build the dishonest case he knew was being forged on the other side of the Atlantic.

There is a lot still to be discovered. I believe we need to know exactly what happened in 2002 in order to decide what we are going to do now. The collapse of allied purpose is clear, Iraq is in free fall, yet we still have not found out exactly how a small group of politicians and officials hijacked policy and took us to war against the clear wishes of the nation.

As the situation deteriorates in Iraq, Britain's need to distance itself from Blair's policy increases by the day. We need more answers. The call on the political establishment outside Number 10 is urgent. The House of Commons must show it is not been entirely debauched by party politics and bring the government to account and that includes Labour members.

In the meantime, my mailbox is open all hours for the slightest information that may cast light on the path to war.

henry.porter@observer.co.uk