Mr Blair should understand the damage inflicted on his reputation

If evidence of WMD is not found, it could be difficult for Mr Blair to persuade the public of a real threat next time

Let's go back for a moment to Japan last Saturday, the day after Tony Blair heard of David Kelly's death, and undoubtedly the lowest single point of his premiership. No doubt the nadir was that painfully embarrassing press conference at Hokane. Yet the more telling public moment had come several hours earlier when Blair made an extraordinarily pro-euro speech to an audience of Japanese businessmen, delivered, understandably given the circumstances, with all the passion of a Speak Your Weight machine. A speech whose text took an unmistakably macho sideswipe at an argument indelibly associated with Gordon Brown - namely that a reason for staying out of the euro was the "perceived rigidities" of Britain's European trading partners.

"This is a strange way of casting the national interest," read what, in any other circumstances, would have been an electrifying passage. "Since when did a successful businessman withdraw from a market because he feared that his rivals could not compete?"

The reason that Blair, still reeling from the shock of the news he had received hours earlier, didn't actually say these words isn't, and perhaps never will be, clear. Maybe in his nervousness his eye simply skipped over them. But maybe he missed them out because, subconsciously or not, he felt they suggested a power and authority over his government that he plainly did not at that moment possess. The issue of the euro itself is wholly irrelevant except in so far as it happens to be central to the Blair project. The point is that it has never been more difficult than at that moment to imagine him persuading a reluctant British people to vote yes in a referendum.

So this is certainly Labour's Westland. It's not just the eerie similarities of detail. A woman in charge of departmental press relations is ordered, in bitterly controversial circumstances, to make public an explosive piece of information in the interests of her political masters (for the DTI's Colette Bowe and the Attorney General's advice at that time read the Ministry of Defence's Pam Teare and the confirmation of David Kelly's name now.) There are unmistakable signs, as there were then, of an unseemly scramble between the parties to fix the blame on each other. And an enquiry was set up.

There is also the larger context. As that moment on Saturday suggested, this is Blair's weakest, most vulnerable moment, as Westland was Thatcher's. Thatcher recovered, of course, to lead her party triumphantly through the 1987 election, blessed - as Blair is today - by a hobbled Opposition. But this is offset by the fact that this is more serious still than Westland, involving as it does a war and the death of a public servant rather than the future of an obscure helicopter factory.

Despite the limitations on the remit of Lord Hutton's enquiry, the war is inextricably part of this story. For what the David Kelly tragedy crystallises in the public mind is the realisation that, whatever the details of how he was reported, treated and unmasked, the doubts he expressed about the use of intelligence to justify the war have a validity which was not apparent to the House of Commons by the time it crucially voted its support, when the Prime Minister was personally appealing to MPs to trust him on the threat. And when this was probably the first war of its scale fought on the basis of intelligence alone.

In Britain this isn't really just about the 45 minutes it supposedly took Saddam to fire his weapons, or in the US about the 16 words in the President's State of the Union speech about Niger. It is that the primary public justification for making war on Iraq - the justification that the Government strained every sinew of its strength to persuade the UN and the people to accept - namely that Saddam represented a clear and present danger to the interests of the West, has also turned out to be the most doubtful. The public has rumbled this.

In hindsight, it looks as if another course was possible. This was to play it longer and to be much franker, from much earlier on, about the fact that it was impossible to be sure how urgent was the need to deal with the threat, but to say that Saddam had consistently flouted UN resolutions and that sooner or later this had to be dealt with. That too would have had consequences, one of which would have been the paramount need to wait for the second UN resolution which Blair repeatedly gave his MPs every reason to expect, even if it required a costly process of troop rotation through the summer as the inspectors were allowed to continue their work. It was this which the US, and its British ally, refused to do.

The loss of public trust which flowed from all this is not just an academic question. Trust in a Prime Minister in the face of threats which could lead to war is one of the highest - some would say the highest - imperative of all. It's not too much to say that crying wolf on one occasion may actually encourage real threats on others. In the admittedly very different circumstances of the Falklands, it now looks as if General Galtieri was persuaded to invade the islands because he thought that Margaret Thatcher was too divisive a figure to persuade the British people to back a war. He was wrong, of course. But if clear evidence of usable weapons of mass destruction is not found, it could be more difficult for Tony Blair to persuade the public of a real threat next time, if there is one.

Such a discovery would make a big difference of course. So would the provable death, or even better the capture, of Saddam as well as of his sons. So would the evolution - at present still a seemingly distant prospect given the present conditions of guerilla war - of Iraq into the peaceful, democratic and prosperous state which Tony Blair has consistently promised, most recently in his Washington speech. So too would some concrete evidence that the world, and not just the Middle East, is actually a safer place as a result of the war.

No one sensible is saying that Saddam's removal from power was other than an unalloyed benefit. But unless and until those other conditions are met, Mr Blair is going to find it more difficult to restore the political capital he has expended. When Chinese students can call him expertly to account in television pictures that flash instantly around the world, this is an international problem too. Blair's apparent recovery in confidence as his round-the-world tour schedule was remorselessly fulfilled this week testifies to the same world-class resilience in the face of crisis that Thatcher had. His government still enjoys poll ratings the Tories would have killed for in a period when when they still serially won elections. But whether Mr Blair is right to imply that "history will forgive" him even if WMD are not found, a large section of the British people are going to need more evidence of the benefits of war before they, too, are ready to do so.