I do understand the Prime Minister's mind, but I still think he was wrong to go to war

Mr Blair is prepared to ditch rules the world has found workable for 350 years without anything to replace them

The Prime Minister has called for a sort of truce with his Iraq critics. In the important speech he made on Friday, he said in effect, let's take each other seriously. If I concede that your arguments opposing the war are perfectly reasonable, even though I don't agree with them, in turn "believe your political leaders or not, as you will. But do so, at least having understood their minds".

Thus much more fully than ever before, Tony Blair showed that he comprehended the force of the assertion that Iraq posed no direct, immediate threat to Britain. He can see why people said that Iraq's possession of weapons of mass destruction, even on the Government's own case, was not serious enough to warrant war unless there was a specific UN resolution.

He also took on board, likewise without approval, his critics' clinching argument that Saddam Hussein could, in any event, be contained. Yes, these are perfectly respectable views, Mr Blair now says, but wrong. This itself is a great improvement. Hitherto the Prime Minister has given the impression that he regarded his opponents as no better than appeasers.

The divergence between Mr Blair and his critics goes back to the 11 September attacks on the United States. "What galvanised me," Mr Blair said on Friday, "was that it was a declaration of war by religious fanatics who were prepared to wage that war without limit. They killed 3,000. But if they could have killed 30,000 or 300,000 they would have rejoiced in it." The key word here is "war".

Many who disagree with the Prime Minister thought at the time that using such language was an error. What the West had experienced on that terrible day was a great crime. It followed that the perpetrators of what was mass murder needed be hunted down aggressively across the world as criminals not as irregular soldiers. Indeed misdescribing 11 September was the first step towards a misconceived response. The "religious fanatics" weren't engaged in war; they were engaged in crime. War always involves a struggle for control of territory.

On the contrary, argues Mr Blair, "we are in mortal danger of mistaking the nature of the new world in which we live. Everything about our world is changing: its economy, its technology, its culture, its way of living. If the 20th century scripted our conventional way of thinking, the 21st century is unconventional in almost every respect. This is true also of our security." Now I positively enjoy unconventional thinking. "Off the wall" appeals. But I don't wish British troops to be sent to war on the basis of virtual reality.

In what way has our security situation changed? Because, argues Mr Blair, weapons of mass destruction are being traded right round the world and can fall into the hands of terrorist groups. These fanatics will stop at absolutely nothing to cause death and destruction on a mass scale. We know, too, that there are states that will trade the technology and capability of such weapons. "It is time that this trade was exposed, disrupted, and stamped out." Indeed but examine the links here.

If only it was as simple as rogue country A supplies terrorist group B with weapons of mass destruction so that it can attack a British target and as a result we may properly attack rogue country A in our defence. But reality is never so simple. To give an example, conventional arms manufactured by each of the five permanent members of the Security Council circulate from one terrorist gang to another. Yet we don't blame the original suppliers for the subsequent outrages.

This lack of clearly identifiable culpability is why the Prime Minister is forced to carry his argument a stage further. He states that the traditional principle of international relations which has held sway since the treaty of Westphalia in 1648 is no longer appropriate. This doctrine holds that a country's internal affairs are "for it and you don't interfere unless it threatens you, or breaches a treaty, or triggers an obligation of alliance". But, added Mr Blair: "I did not consider Iraq fitted into this philosophy."

But nor does Iraq fit into Mr Blair's new way of thinking. To be justifiable, pre-emptive action requires accurate intelligence of what is happening within so-called "rogue" states. Notoriously this was not the case in Iraq. It represents one of the most spectacular intelligence failures of all time. Was that because of poor technique or because the task is inherently too difficult?

More unfortunately, the invasion of Iraq has had perverse results. It has already caused the death of more than 500 coalition troops and thousands of Iraqi civilians, with no end in sight. Moreover, in the aftermath of the war, such weapons of mass destruction as Iraq may have possessed could be pilfered on a wide scale. The invasion has also inevitably generated resistance to an occupying force, thus giving al-Qa'ida the warrior status it had previously lacked. It may yet lead to a civil war whose miseries could exceed the worst that Saddam Hussein could do to his people.

The war crimes committed in former Yugoslavia provide a frightening example. When the Prime Minister states that at least the Iraqi people are better off without Saddam, I remain unconvinced. The metaphor linking the frying pan with the fire might have been invented for this situation.

Worse still, the Prime Minister is prepared to ditch the Westphalia rules that the world has found broadly workable for 350 years without having any new system to put in their place. The absence of rules is anarchy. Post-11 September, the world has slipped closer to lawlessness than at any time since 1945. Mr Blair knows this very well. In his speech he said he understood the concern the international community has over Iraq. "It worries that the US and its allies will by sheer force of their military might, do whatever they want, unilaterally and without recourse to any rule-based code or doctrine." Exactly.

What, then, is to be done? Here we come to the weakest part of the speech. All that the Prime Minister could offer was reform of the United Nations - so that "its Security Council represents 21st-century reality" and so that it has "the capability to act effectively as well as debate". Yet equally the Prime Minister knows that the chances of such reforms being agreed by the five permanent members of the Security Council let alone by all the member states is close to zero. This is a serious problem.

The United States as the world's sole superpower could declare some rules governing pre-emptive attack to cover itself alone. That would add a small degree of predictability to international relations but would be otherwise worthless. It would only prove that, after all, the Treaty of Westphalia was going to have to do some more decades of service.

Within the terms of the truce, then, I believe I well understand the Prime Minister's mind. Yet I remain unconvinced. Not just me, either. Public opinion would not support another Iraq-style war. Mr Blair's unconventional thinking is going to remain just that - unconventional.