The Prime Minister remains safe while Mr Duncan Smith leads his opposition

Not for the first time, Tony Blair should send a thank-you note to Iain Duncan Smith. There was Mr Blair in the Commons yesterday, still without the discovery of a single weapon of mass destruction to protect him, his senior ministers screaming about rogue elements in the security services and former cabinet ministers alleging that he had duped us all. That is quite a political cocktail. Mr Blair was vulnerable.

At least that was how it seemed until Mr Duncan Smith rose to face his open goal. Somehow or other - it is a form of genius - the Conservative leader managed to ask several questions that Mr Blair was able to dismiss unequivocally. No, Mr Blair insisted, he was not questioning the integrity of the intelligence service as a whole. Yes, he was raising questions about the source of the Today programme's story, the one who alleged that Downing Street had "sexed up" intelligence material. No, Mr Blair did not know who the source was. That was the whole point. He wished he did know who it was.

For once Mr Blair's answers were clear and transparent. At which point Mr Duncan Smith rose majestically and accused Mr Blair of being equivocal. He had lost it. Mr Blair looked more content than a liberated Iraqi. For the time being he was free.

Looking at the doleful Conservative MPs behind him Mr Duncan Smith had managed - again not for the first time - to turn a crisis for Mr Blair into a leadership crisis for his own party. What was even more remarkable is that the Leader of the Opposition had been so off-beam that he had allowed Mr Blair to wallow in his unequivocal responses. As we shall see, it is the prime ministerial equivocations relating to the war that have landed him in trouble.

What a shame Ken Clarke is not Leader of the Opposition at this moment - a terrible shame for the Conservatives, because they would be ahead in the polls now. More importantly, the lack of an effective Leader of the Opposition is a loss for British politics as well. At a time when there are many unanswered questions about Mr Blair and his role in the build-up to war, Mr Duncan Smith, as an ardent supporter of the conflict, cannot pose them. This is worrying for all of us: voters are deeply sceptical about what happened in Iraq. They need to hear their questions asked, to sense that their doubts are being fully voiced by elected representatives.

Mr Clarke is in the fortunate position of having been bold enough to oppose the war. He is therefore able to ask the key questions: when did Mr Blair decide to take Britain to war, and when did he make such a commitment to President Bush?

It is astonishing that no one knows the precise answer to these questions, but the public equivocations from Mr Blair suggest that it was months and quite possibly a year before the military action took place.

At their joint press conference after the summit at Crawford in March of last year, Mr Blair stated equivocally: "Iraq's weapons of mass destruction must be dealt with. How we deal with them is still to decided, but that they must be dealt with is beyond doubt." This was an extraordinary statement. Privately, senior Foreign Office officials had reached the same conclusion as Robin Cook, that containment and deterrence were working. Mr Blair was stating emphatically that this was not the case. What did Mr Blair say in private at that summit? I cannot believe that the prospect of military action was not discussed, given that they had both reached the conclusion that the weapons "had to be dealt with".

Last July, when journalists asked questions about Iraq, Mr Blair insisted that they were "running ahead" of themselves. This implies that he was working to a timetable of sorts, although not the one shared by the journalists. In September he offered a defining equivocation when discussing the role of the UN: "The UN must use this as an opportunity to address the issue of Iraq's weapons, not as a way of avoiding the issue." I wrote at the time that this statement could only have one meaning: "The UN must back our approach or else we will attack Iraq anyway" - which is precisely what happened when the UN did not agree to back a second resolution.

This suggests to me that Mr Blair had convinced himself of the need for military action - preferably backed by the United Nations - by September, and quite probably before then. As Mr Clarke implies, the entire debate over the weapons of mass destruction was determined by a timetable for military action. The reason the UN weapons inspectors were kicked out was not because Saddam had failed to "fully co-operate", a term that was never clearly defined, but because the US and Britain had kept their troops waiting long enough.

Mr Blair is a clever equivocator, but he is not a liar. I do not believe that he invented intelligence information. I have no doubt that weapons of some sort will be uncovered. The only relevant question is whether President Bush and Mr Blair rushed to war and talked up the threat of Saddam's weapons to justify their impatience. This is where the focus of the investigation by the Foreign Affairs Select Committee should lie. Very few MPs opposed military action in any circumstances. Many of them, including some still sitting around the cabinet table, had doubts about the timing and the attempts to secure UN backing for a military timetable that had already been more or less agreed.

As matters currently stand, there is an entirely credible suspicion that the UN inspectors were stopped in their tracks because they would not have found weapons lethal enough to back up the case for an immediate war. In which case President Bush in particular would have faced the humiliation of withdrawing his troops with Saddam still in place. That was never going to happen. President Bush could not have fought the next presidential election with Saddam still in power, having marched his troops to the top of the hill and marched them back down again. So when did Mr Blair agree to the military action, and did that lead him to convince himself that the threat posed by Saddam was greater than it really was?

There are so many related unanswered questions, and it should be said, to reassure those who despair of the House of Commons, that they were all posed in a subsequent debate once Mr Duncan Smith had left the chamber: Was some of the intelligence material simply wrong? Did Mr Blair suspect that it might be wrong, but still published it to advance the case? If Mr Blair is convinced lethal weapons are still in Iraq, why were they not uncovered as matter of urgency given the risk that they could end up in the hands of terrorists still roaming the country?

But while Mr Blair's main parliamentary interrogator is Iain Duncan Smith, he will remain untroubled by his weekly bouts in the House of Commons. That is the moment when the equivocator becomes confidently and triumphantly unequivocal.

s.richards@independent.co.uk