It's a fun day for Sir John Chilcot, because he will get to meet both Nick Clegg and David Cameron to talk about his inquiry into the Iraq War. He will find them, I am sure, wanting nothing that is not best for the country, or that would be unconducive for the learning of important lessons.

And I hope, given that I've put his name in the first paragraph and that this is, after all, The Times, Sir John will also read this article, urging upon him that he hold the entire inquiry in public, despite my certainty that it will, in one important sense, do no good, because it will change no one's minds.

The bit that won't happen is the supposed “truth and reconciliation” element in which a cynical public is satisfied that - at last - there has been an accounting. This is impossible. Some of the most exalted and popular opponents of the war are implacable in their interior knowledge of the wrongness of the conflict and of the perfidy that led up to it. No facts or interpretations that they could possibly hear would ever change their minds. Instead, they await the unlikely moment when their beliefs are demonstrated, by some hidden memorandum or mandarin testimony, to be utterly and irrefutably correct. Then, perhaps, they will get the Trial of Tony Blair for War Crimes that they have been wanting for so long - the final scratch to their intolerable itch.

I was reminded about this implacability when the splendid broadcaster Jon Snow put in a cameo appearance at my session on conspiracy theories at the Hay Festival last month and delivered the opinion that there had been a real conspiracy to get us into the Iraq War, and that I had been one of the dupes of the conspirators.

Just last Sunday the amnesiac inventiveness of critics was demonstrated when one newspaper excitedly detailed a memo that it had seen of a 2003 meeting between George Bush and Mr Blair, adding that “Gordon Brown's critics fear a closed inquiry will also black out embarrassing truths. Today's revelations will only fuel the fire.” In fact the memo - which was very selectively quoted - had been “revealed” on Channel 4 News and in The New York Times three and a half years earlier.

For six years now - and, of course, it can be understood - the war's critics, unconfronted with the alternative reality that their preferences would have bestowed upon the world, have had it all to themselves. The war was “immoral”, “illegal” (so why no prosecutions in all that time?) and fought under a “false prospectus”. Claims of up to two million dead in Iraq have been bandied about and believed. Any of the inquiries into events leading up to the war have been dismissed as whitewashes, essentially for failing to give the answer that critics want; that answer being that there was a deliberate and wicked attempt to fool the peoples of America, Britain and the world into war.

The Hutton inquiry, of course, wasn't about that. But critics wanted it to be and when - as had seemed to me fairly inevitable - Lord Hutton (all of whose evidence was heard in public) criticised the BBC for running a wrong story and refusing to correct it, he was excoriated. Often by people who had never (and still have never) read his report.

Then came Lord Butler of Brockwell, who looked at intelligence failures in the run-up to the war. He did criticise the Government, and how its “informality” had “reduced the scope for informed collective decision making”. But this is what Lord Butler's committee said about the evidence: “We have reached the conclusion that prior to the war the Iraqi regime... had the strategic intention of resuming the pursuit of prohibited weapons progammes, including, if possible, its nuclear weapons programme... In support of that goal [it] was carrying out illicit research and development and procurement, activities... [and it] was developing ballistic missiles.” Not whitewash maybe, but “mandarin understatement” said the more intelligent critics.

Lord Butler's was the consensus in 2001-03, or as Sir Menzies Campbell put it on publication of the September 2002 dossier: “We can also agree that Saddam most certainly has chemical and biological weapons and is working towards a nuclear capability.” Or Robin Cook, writing in February 2001: “We must not be deceived. Saddam still threatens his neighbours. Unchecked, Iraq could develop offensive chemical and biological capabilities, and develop a crude nuclear device in about five years.”

So the “lies” weren't lies at all, leaving just one extant charge of culpable dishonesty - that Mr Blair and Mr Bush secretly decided on war in 2002, come what may. I went into this at length with those I interviewed for a series on the Blair premiership in late 2007. Sir David Manning, his foreign affairs adviser and later Ambassador to the US told me that Mr Bush had agreed with Mr Blair that, were Saddam to comply fully with international obligations, there would be no need for invasion because they would have effectively “crated the guy”. There was no prior hidden compact.

The irony of the discussion on an open inquiry, is that I think Mr Blair would probably be the star turn, pointing out some of the above and that, as a consequence, critics would declare another whitewash. And, though this would be a waste of time, at least we'd be able to tell Jon Snow and others that it had been done.

But a serious inquiry could help with two things. The first seems to be specifically precluded by the time frame given to Sir John, and that would be to investigate whether Britain had, from 1980 on, effectively encouraged Saddam's belligerence towards his neighbours. This constitutes the unheld inquiry into the First Gulf War.

The second is the area where Sir John should concentrate, which is on how postwar planning in Iraq was done, how the wrong things were anticipated and the right things overlooked, how a fantasy of policing and governance was constructed, and why warnings from those on the ground were ignored. And whether such mistakes are inevitable or could be avoided. This is what we really need to know, and what might save lives in the future.

What Sir John should also tell Mr Cameron and Mr Clegg is this: you can have open or you can have quick, but you can't have both. If you are going to get dozens of folk going on the stand in full view, they will have to prepare, and natural justice demands they are allowed to do so. Quick and open may be gaudy politics but it is bad inquiring.

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