May 1, 2005 - 'I would never do anything against the advice of the Attorney General," the Prime Minister has repeatedly insisted. Yet he did do something against the Attorney General's advice. He helped the Americans invade Iraq and replace its Ba'athist regime.

Lord Goldsmith's confidential advice to the Prime Minister on the legality of invading Iraq without a second UN resolution, revealed for the first time last week, was equivocal about almost everything. It was clear about one point and one point only: "Regime change," insisted the Attorney General, "cannot be the object of military action." Any invasion which had that goal would be unambiguously illegal under international law.

As everyone knows now, and knew perfectly well then, the whole point of the invasion of Iraq was regime change. President Bush said so with Tony Blair standing by his side. At the joint press conference the two leaders gave at George Bush's ranch in Crawford, Texas, on April 6, 2002, he helpfully summed up the purpose of the military action against Iraq in a single sentence: "We support regime change." For George Bush, no further elaboration was necessary.

Despite Mr Blair's statement that "If I had not taken the action I did, Saddam would still be in power," it was not he, but President Bush who removed Saddam. Tony Blair was an accessory to the American invasion - but British forces were so small a part of it that they were not critical to its success. As President Bush emphasised, the Americans were prepared to go it alone without Tony Blair and Britain's armed forces. Two weeks before the invasion started, he offered to do precisely that, as he saw the enormous political difficulties the Prime Minister faced in getting Parliamentary approval for the war. Tony Blair rejected the offer. "I said I'm with you and I mean it," he insisted.

Last week, most of the discussion of the Attorney General's advice centred on whether it was "arguable" that the invasion would be legal without a second UN resolution explicitly authorising it. Yet when compared with the glaring illegality of an invasion whose explicit purpose was regime change, this is a non-issue. The Prime Minister knew it, the Attorney General knew it, the Cabinet knew it, and so did anyone who bothered to think about it. What, then, was Tony Blair doing when he stated: "What is important is that whatever action we take is done in accordance with international law"?

President Bush never wavered in his conviction that regime change was the main purpose of the invasion. He was not interested in a second resolution from the UN - he was not really interested in the question of whether America's action was in accordance with "international law". It was the Prime Minister who persuaded him to try to get a second resolution from the Security Council explicitly authorising war. President Bush made the effort as a favour to Tony Blair, despite the very evident scepticism of Messers Cheney and Rumsfeld.

Their scepticism was understandable. Returning to the UN was a gesture whose only point was to keep up a charade for the benefit of the Labour backbenchers. Tony Blair knew that if he were honest with them he would fail to get them to vote for the war. The pretence, however, did not fool the members of the Security Council, which never came close to approving that second resolution. They shared the Attorney General's original view that an invasion whose purpose was regime change could not be legal under international law.

Mr Blair's attempt to claim that the aim of the invasion was really only to disarm Saddam - if it happened to topple him from power, that was an unexpected side-effect - was pathetically unpersuasive. Tony Blair was like a man who offers as his defence to a charge of being an accessory to murder that, although he had joined a group of men whose ring-leader had expressly stated as they set out together that their joint purpose was to bludgeon a rival to death, he had gone along only because he wanted to disarm the guy: bludgeoning him to death was simply a means to that quite separate goal.

The repeated insistence of the Prime Minister, the Attorney General and, indeed, the whole Cabinet, that the invasion of Iraq was compatible with international law seems simply to have been a pose to try to fool people who genuinely wanted the invasion to be legal - but were too stupid to see for themselves that it wasn't.

Of course, a number of the people in that category include the Labour MPs who voted for it after hearing the Prime Minister and the Attorney General assure them that it was in line with what international law required. Many of those MPs are now very angry at their own stupidity. They have deflected their anger on to the Prime Minister and the Attorney General for not sharing the confidential advice that was released last week.

This is simply a further demonstration of their own idiocy, for there was nothing in that confidential advice that was not obvious at the time of the Commons vote. It was extremely easy to work out that the invasion was incompatible with international law. If that law has a guiding principle, it is that the invasion of one country by another in order to replace its ruler is a flagrant violation of state sovereignty.

The Prime Minister's actions show that he has never regarded "conformity with international law" as critically important. He has, in the past, been quite willing to get involved in military expeditions that did not have explicit UN approval - the test for their acceptability under international law. The toppling of the regime in Sierra Leone, the bombing of Kosovo and the bombing of Saddam in Operation Desert Fox in 1998 are all examples, and ones of which he insists he is proud.

Clare Short and Robin Cook, two ministers who resigned over the illegality of the Iraq invasion in 2003, were enthusiasts for the equally illegal Desert Fox in 1998. But in that case, a Democrat - good old Bill Cinton - was in the White House. In 2003, its occupant was a Republican. To get over Labour MPs' qualms about associating themselves with George W. Bush, Tony Blair had to pretend that international law licensed America's military action.

I do not believe that the Prime Minister cynically exploited his MPs' stupidity by his pretence: the first person he fooled by his sophistry was himself. A self-deluding sophist is not an ideal character to have as the leader of our country: it is what we will get if we re-elect Tony Blair and Labour next week.

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