For nearly a year the great faultline under the Blair premiership has been the Attorney General's opinion on a war of exemplary pre-emption in Iraq without a further, specific UN resolution authorising any necessary means to force Saddam Hussein to comply with previous UN resolutions. Government lawyers were riven by this question. The number two in the Foreign Office legal department, Elizabeth Wilmshurst, resigned on the spot when she read the Goldsmith advice.
The Attorney General, the Prime Minister and his ministerial group in Iraq have gone to immense pains to ensure that the provenance of that opinion, as well as its full text, was not divulged to either Parliament or the public. If Katharine Gun's trial had proceeded it would have been very difficult for the prosecution to have prevented the defence having access to this legal opinion in full. That opinion turned largely on what intelligence told Whitehall about the degree to which Saddam Hussein and his weaponry posed a danger to other people's nationals other than this own.
If the Butler inquiry into intelligence does not follow the trail of intelligence documents into the Attorney General's office prior to the making of the legal opinion they will have let themselves and their country down.
In many ways, Lord Butler is an admirable choice for the job. A Whitehall lifer who served no fewer than five prime ministers in senior positions (Heath, Wilson and Thatcher as a Number 10 private secretary; Thatcher, Major and Blair as Cabinet Secretary), it's not so much that he knows where the bodies are buried in Whitehall, he was the body. Lord Butler, too, is a traditionalist believing in proper demarcations between the functions of Britain's governing tribes - ministers, crown servants and special advisers. He believes in proper process.
But now Robin Butler has to break the habits of a lifetime if his report is to have any credibility when it appears at the end of July. This would have been the case even if the Hutton report had not persuaded the political market (wrongly, in my view) that the Butler report is bound to be another executive-minded production that will cause the incumbent in Downing Street little anxiety. The announcement that the Butler committee of privy counsellors will concentrate on intelligence systems rather than individuals has reinforced people's minimal expectations.
In the end, this will do neither Butler et al nor Tony Blair and those who served him as warrior-premier any good at all. Because posterity will not be fooled. Lord Butler and his team should behave less like systems analysts and more like scholars, working as war historians rather than technocrats of state. In other words, from the first meetings and reading sessions in the Cabinet Office (the security-swept Joint Intelligence Committee room has been set aside for them), the Butler group must apply the skills of forensic scholarship, and bust their terms of reference and reconstruct as fully as they can all the real-time factors, files and discussions that went into the decision-taking.
Above all, they must not neglect the crucial intelligence inputs into the central opinion by Lord Goldsmith that the war was legal without another specific United Nations resolution. The Butler committee members cannot shirk this specific task if they are truly going to examine the uses to which Iraq-related intelligence was put.
How can he do this? My research historians 30 years on would start this way - by trawling for every scrap of material, from the Joint Intelligence Committee assessments to the tersest e-mail from, say, Number 10 press officers who should not have been allowed anywhere near the ebb and flow of the internal intelligence debate. Above all, historians will want to make sure they get at the Prime Minister's supply of personal documents - the stuff that went into his special intelligence box each day (which used to be known as "Old Stripey" because of its distinctive blue leather bisected by a red stripe).
If Robin Butler does not "rise to the level of events", to use a phrase beloved of the late Roy Jenkins, we have one last hope. If the Liberal Democrats had agreed to nominate a member of the Butler committee it would have been Alan Beith MP, who chairs the all-party Commons Select Committee on Constitutional Affairs. This scrutinises, among other things, the Law Officers' Department. It could (and should if Butler won't) investigate the provenance of the Attorney General's opinion on the legality of the war. Its first witness? Lord Goldsmith himself. Its second? Elizabeth Wilmshurst.
If Lord Butler declines to perform his historic function, Parliament is our last best hope of discovering why our armed forces were used in the 2003 war of exemplary pre-emption and whether, in international law, it could be justified.
The author is Attlee Professor of Contemporary British History at Queen Mary, University of London.
This article is based on a piece in the latest edition of 'The Tablet'.