At the heart of the row is the murky question of whether Britain required a specific United Nations resolution, authorising all necessary means to send in the troops – or whether existing resolutions sufficed.
The Foreign Office, it is clear, had no doubts. Sir Michael Wood, its senior legal adviser, told the inquiry on Tuesday that he had warned foreign secretary Jack Straw that, without a proper legal basis, invasion would constitute a "crime of aggression, one of the most serious offences under international law". War would require "express authorisation" from the UN, Sir Michael added.
Britain could not claim that it was acting in self-defence, nor for humanitarian reasons, he said. There was "no doubt" about it.
That put Mr Straw in the frame. Documents released by the inquiry reveal him telling Sir Michael that he "did not accept" the advice. He even went so far as to complain that he was being "dogmatic" in what was never a black-or-white situation. Under the spotlight, Mr Straw moved quickly to shift it yesterday.
"Advisers advise, ministers decide," he said, in a statement, before adding: "The simple truth is you get a range of advice on legal issues always. The ultimate decision was always going to be the attorney general's."
That put Lord Goldsmith in the hot seat. Having spent six months arguing that war would be illegal without a specific UN mandate, the former attorney general suddenly performed a volte-face, at the very moment the government required it. He insisted yesterday it was "nonsense" to suggest he had succumbed to political pressure. And he then flung the buck across the pond.
His U-turn had been prompted by evidence garnered from the Americans which, they claimed, showed how there was no legal veto to military action under the existing UN resolution 1441. It was this that persuaded Lord Goldsmith to change his mind on legality, he claimed yesterday.
This suggests that, effectively, it was Condoleezza Rice who provided the legal basis for Britain's involvement. Sadly, however, neither Ms Rice, not anyone else from the former Bush administration, is going to be called for evidence.
The inquiry tomorrow questions Tony Blair. While he is sure to be asked about the same issues, the fact is there was nothing in Lord Goldsmith's evidence which proved beyond doubt he was pressured by the PM into his volte-face.
The buck for that particular decision has been passed all the way to Washington.