It is risky to attack Saddam; but it is much more risky to leave him alone

Tony Blair is a charming chameleon. As such, he usually finds it easy to conciliate his visitors; he can almost always convince them that he agrees with what they were saying. Once upon a time, he even persuaded Paul Johnson that a Blair government would make Margaret Thatcher seem like a Social Democrat; Mary Whitehouse, a sixties trendy; Enoch Powell, a Euro-federalist.

King Abdullah of Jordan is the latest victim of the Blair charm. The King left Downing Street believing that Mr Blair shared his doubts about the wisdom of invading Iraq. If that is so, Mr Blair has been deceiving all his senior diplomatic advisors, who have no doubt that he intends to support the Americans. He has also been deceiving the Ministry of Defence, which has ordered a number of battalions and other units to prepare for war. Above all, he has been deceiving the Americans, who are certain that they not only have the UK's support, but the UK's enthusiastic support.

The PM is, of course, aware that a war will be unpopular in the Labour Party. This does not worry him; he has never paid much attention to Labour MPs' views. He has been telling senior civil servants that there will only be a handful of ministerial resignations plus around 50 inconsolable backbenchers. Given the size of his majority – reinforced by parliamentary support from the Tory Party on any war votes – that is not a life-threatening revolt. Some commentators believe that Mr Blair is underestimating the extent of the dismay in Labour's ranks. But even if that is so, Tony Blair knows how to control his own party. He will not care whether his MPs are grumbling in the bars, as long as they do what they are told in the division lobbies. He is set on his course.

And so he should be. There are overwhelming reasons for destroying Saddam. The first and greatest is the man's evil, and capacity for evil. Saddam began his career as a brutal egomaniac, and his good qualities have receded with age. For the past two decades, he has regarded Iraq solely as a vehicle for self-aggrandisement. He has inflicted unimaginable sufferings on the Iraqi people and on their neighbours – and he has always sought the means of inflicting more. From the outset, Saddam has been striving to acquire weapons of mass destruction.

The Israelis had the wisdom to abort his earliest efforts, by destroying the Osirak nuclear reactor in 1981. But as long as he is in a position to exploit Iraq's industrial and financial resources, the danger persists. He already possesses chemical and biological weapons, including anthrax and botulinum. It is not easy to design a delivery system for biological weapons, but a fanatic with a suitcase, luck at border points and a lack of interest in personal survival could pose a terrible threat. Once Saddam realises that he is finished, we can expect him to strain every molecule of his malice. In the Führer-bunker, with his foes closing in, Hitler dreamt of hideous miracle weapons. Saddam may be able to use them. We in the West will be fortunate if we can intercept all his attempts at revenge.

But this is not an argument for declining to provoke him and persisting with the policy of containment. An uninvaded Saddam would be no less malignant; he would merely be more powerful. There is no guaranteed method of containing biological or nuclear weapons.

The Israelis understand this and would not hesitate to take pre-emptive action to stop Saddam deploying weapons that could destroy their country. There are obvious dangers in the West acting to destroy Saddam; it could have destabilising repercussions throughout the Arab world: How much more so, were the Israelis to use their weapons of mass destruction to destroy his.

The West has to act urgently and decisively. President Bush understands this as does Premier Blair. In a favourite phrase of a previous prime minister: there is no alternative.

Even if the PM had doubts, which he does not, there would be a good case for suppressing them. As things are, and because of Britain's wholehearted support since 9/11, the special relationship has never been in better shape. The Americans trust us. George Bush and his team always knew they could rely on a Tory PM. They now find that they can also rely on a Labour one. So at a time when many Americans have come to hold the continental Europeans in contempt, they find that they have a commonality of worldview with Britain. This has had a profound effect on attitudes in Washington.

If Mr Blair had expressed reservations from the outset, and had made it clear that America would not be able to rely on British military assistance in the war against Iraq, he would have met a cold response in Washington. But this would not necessarily have been fatal to Anglo-American relations. Allies are allowed occasional lapses into disagreement.

But if Mr Blair were to decide to renege on the Americans at this late stage, after long months in which he had assured the Americans that he was soldier to soldier with them, the American response would not merely be cold; it would be refrigerated, and the entire infrastructure of UK/US relations, built up over many decades, would be in jeopardy. There would be an end to intelligence co-operation; no more of those documents, which regularly fascinate senior British politicians, marked "for US and UK eyes only". The special relationship might have recovered from an initial British lack of support, it could never recover from a later withdrawal. As it is, we do have diplomatic influence in Washington, which could be used, especially over the Palestine question.

The Americans claim to be committed to a Palestinian state, but the latest bus bomb atrocity will only strengthen the position of those who are viscerally opposed to all dealings with the Palestinians. The British will need to add their discreet voices to the covert pressure of those members of the Bush administration who insist that, despite the faults of leading Palestinians, the Palestinian people have an unanswerable moral case in their search for nationhood. But if Mr Blair broke ranks now, the British voice would no longer be heard.

A war against Iraq is not a risk-free enterprise; we are entering a most dangerous phase of world history. But the dangers of action are as nothing to the dangers of inaction. It is risky to make war on Saddam, because he is a heavily armed, bloodstained tyrant who cares nothing for the welfare of the human race. For all those reasons, it would be even more risky to leave him alone.

Mr Blair knows this; perhaps he would have been wiser to share his knowledge more widely. He obviously calculates that, as part of his strategy for dissension within the Labour Party, it would be better to take his MPs by surprise. He ought to consider whether it might not be more fruitful to speak over the heads of his MPs and address the country directly. If he takes the country with him, he has nothing to fear from his own party.