Myth meets reality on Wednesday. The common view that Tony Blair is a lying warmonger will collide with one of the most thorough investigations of recent history.

I don’t know what is in the Chilcot report, but I do know what it won’t say. It won’t say that Blair made up claims about Saddam Hussein’s weapons of mass destruction to justify military action. It won’t say that he lied. And it won’t say that he is a war criminal.

The reason it won’t say those things is that they are not true, as anyone who had paid any attention to the Hutton report, the Butler report and the Chilcot hearings would know.

The word “Chilcot” will, at 11am on Wednesday, change its meaning. Having been shorthand for “I hate Tony Blair” for seven years, it will henceforth mean “whitewash”.

In fact, if anyone is interested in the real history of what happened, rather than the psychology of a nation’s curious relationship with its most electorally successful prime minister, I suspect that the Chilcot report will be critical of Blair, his Government and the military.

I do not see how it can avoid concluding that the decision to join the US-led invasion of Iraq was a bad idea. From the questions the inquiry panel asked witnesses, it is likely that the report will conclude that military action was unjustified because it was not a last resort.

But it will be interesting to see how it deals with the problem of hindsight bias. It will presumably criticise the intelligence agencies for failing to consider the possibility that Saddam Hussein had actually got rid of his chemical and biological weapons – although it would always have been wise to assume that he hadn’t. What matters is whether it will allow that the decision for military action was a reasonable one on the information available at the time.

Blair hints he could reject the findings of the Chilcot inquiry

The first half of the report, on the build-up to war, is all that Blair’s critics have come for, and most of them will be disappointed, but the second half, on the occupation of Iraq, will probably contain the harsher criticisms. Military leaders are likely be criticised for their enthusiasm for war and their blithe assumption that they would be able to handle the task of running, in the British case, the southern half of a country of 25m people.

Blair and his ministers – including Clare Short, who resigned as International Development Secretary after the invasion – are likely to be criticised for sending in civilian administrators with next to no idea how they could do their impossible jobs.

More broadly, they will be criticised, not for assuming that the Americans had a plan for the aftermath of the invasion, because they knew the Pentagon had no such thing, but for assuming that such a rich and powerful country would be able to sort it out.

If the Chilcot report is about the lessons learned, one of its unwritten conclusions might be to re-learn the lessons of Vietnam.

As someone who thought that the Chilcot inquiry was, politically, a terrible idea, because it gave the unholy alliance of the Labour so-called left and the Tory press the chance repeatedly to recycle an ever-simplified fairy story of the wicked Blair, I hope the report will be a triumph of openness.

The real Tony Blair has always said that he wants all the information to be made public so that people can judge him for themselves. If that is the aim, the Chilcot inquiry has been a huge achievement of accountability and the writing of ultra-contemporary history.

On Wednesday, the inquiry will publish a 2.6m-word report together with several hundred documents, including Cabinet minutes, intelligence assessments and notes from Blair to George Bush and transcripts of Blair’s side of his conversations with the US President. All of it will be on a searchable website.

The Chilcot report is going to be a dramatic breach in the 30-year rule protecting government documents. The rule is currently being reduced to 20 years, but Chilcot will be publishing some documents that are just seven years old (from 2009, when British combat troops were withdrawn from Iraq).