By Paul Reynolds
World affairs correspondent, BBC News website
For some critics of the Iraq war, the Chilcot inquiry will have to set Tony Blair up as a potential war criminal or it will have been a whitewash.
For them, it is now clear that Mr Blair, with President Bush, took a decision to go to war, by July 2002 at the latest, and then manoeuvred to justify and implement that basic approach, failing to get specific UN approval.
Other critics seek a comprehensive analysis of the decisions Mr Blair and his government took (and the publication of all relevant documents) and want the inquiry to make critical comments on those decisions, perhaps even devastating ones.
Given that British inquiries are normally quite gentle on governments (The Franks Report on the Falklands, for example) these critics will be pleasantly surprised if the Chilcot panel uses its claws.
Supporters of the government might hope that Chilcot will vindicate their former leader.
But they are realistic enough to know that the best they can probably expect is that the inquiry will conclude that, as in many wars, the government simply stumbled into conflict, misled in this case by faulty intelligence.
This inquiry is about the political background to the British decision to go to war. It is supposed to say what lessons should be learned.
The Butler inquiry has already looked at the intelligence process. Butler concluded that there were failings and criticised the government for not highlighting warnings about the limits of intelligence in its September dossier on the threat allegedly posed by Iraq.
But it said nobody should lose their job and it was still ready to believe at that stage (2004) that Saddam Hussein might have weapons of mass destruction, perhaps, as it put it "hidden in the sand."
A parallel American investigation, the Robb-Silberman commission, was more outspoken. American inquiries usually are. It used a phrase not found in Butler, concluding that the intelligence agencies had been "dead wrong".
Since Sir John Chilcot was on the Butler panel, it is unlikely that Butler's conclusions will be shifted by this inquiry.
Questions and answers
Answers to many questions will be sought along the way - the decision-making timetable; the use of intelligence; the legal basis for going to war; the advice from the attorney general that security council resolutions short of a specific mandate were sufficient; the planning for combat and the post-war period.
One incident will be worth watching to see the kind of attitude the inquiry adopts towards Mr Blair.
That incident is Mr Blair's reply to the Labour MP Donald Anderson during questions to the prime minister by the select committee on liaison on 16 July 2002 - that is, more than six months before the invasion.
Mr Blair had been suggesting that weapons of mass destruction were the "coming next issues" and said: "The one thing that we have learned post-11 September is that to take action in respect of a threat that is coming may be more sensible than to wait for the threat to materialise and then to take action."
Mr Anderson asked: "Are we then preparing for possible military action in Iraq?"
Mr Blair replied: "No, there are no decisions which have been taken about military action."
He did go on to say that this was "not to say that it is not important that we look at all the various options that we may have but... there are no decisions which have been taken about this yet".
Technically Mr Blair will say that he was correct in that no decisions actually to go to war had been taken but all options would be or were being examined.
However, it has since been revealed, in a leaked Downing Street memo, that only a week later, on 23 July 2002, Mr Blair met his senior ministers, military officers and intelligence officials. At that meeting the assumption appeared to be that there was going to be a war.
The official who wrote the memo concluded: "We should work on the assumption that the UK would take part in any military action. But we needed a fuller picture of US planning before we could take any firm decisions. CDS [chief of the defence staff] should tell the US military that we were considering a range of options."
Chilcot could take a very critical stance towards Mr Blair and his reply of 16 July, especially if it establishes that by then Mr Blair knew he was going to have the 23 July meeting and knew that it would be based not just on the likelihood that a war option would be pursued but that a war was now assumed.
But it could also fall back on a defence of Mr Blair by saying that he was correct in what he said and did not mislead, even if he did not say very much.
This will be an interesting moment.
Will it be tolerant or critical?