Lord Goldsmith had to "express a clear and simple view"
Just as the government relied on the advice of the attorney-general before launching an invasion of Iraq, so the attorney took soundings from some of the country's leading authorities on international law.
Particular weight appears to have been given to the view of Professor Christopher Greenwood QC.
He said at the time that, in his judgment, it was not legally necessary for a second UN Security Council resolution to be obtained.
"Resolution 1441 required the Security Council to consider Saddam's intransigence on the weapons of mass destruction. It did not stipulate that it had to take any further action to legitimate the use of force," said Professor Greenwood.
That is the position that the attorney, Lord Goldsmith, took when he gave his definitive opinion on 17 March 2003.
But we now know that he had not reached that conclusion on 7 March. At that point, there were still grey areas.
So, does it establish beyond doubt that he changed his mind under pressure?
Professor Philippe Sands QC has canvassed the views of a number of fellow barristers and they have concluded that: "This 7 March document is written by a man who, in his heart, recognises that, without a second resolution, the war would be unlawful."
Professor Sands is well used to giving advice to clients, including the government.
"There comes a point where you say 'sorry, I am not willing to firm up my advice, you must accept it with all the qualifications'.
"But the attorney did not do this. I can't tell you why."
Lord Goldsmith has provided a partial explanation in his own response to the leak of the document.
He said the military and civil service needed him to "express a clear and simple view" on the legality of military action.
Matter of interpretation
Herein lies the problem. International law is largely a matter of interpretation and frequently the subject of intense argument.
Professor Greenwood said that the Kosovo intervention also divided lawyers.
"Our London dining club was split down the middle on the lawfulness of Nato action," he disclosed.
And Iraq was even more politically divisive than Kosovo.
So, distilling a number of equally valid interpretations into one unequivocal opinion was, perhaps, always going to be an impossible goal.
The publication of the full March 7th opinion is intriguing for a number of reasons.
It makes clear the strength of the Attorney's concerns about the legal implications of any alleged war crimes committed by UK forces in Iraq.
He is also anxious to stress that UK military action had to be proportionate to the objective.
As he says, regime change, in itself, was not a legitimate goal of the coalition forces.
Yet that is what happened as a consequence of the invasion.