As happens with many long-running series, the latest instalment is the author's darkest work yet. War is approaching; and when it happens, it goes wrong almost immediately. That much we knew from the trailer – Alastair Campbell's heavily edited single-volume diaries, published five years ago. This volume of the complete diaries, taking the story from 9/11 to Campbell's resignation from government in August 2003, completes the psychological thriller.
The biggest omission from the edited diaries was anything rude about Gordon Brown. So here is the missing material about the relationship between Tony Blair and Brown, as it moves into its final phase of total mistrust, hostility and contempt. Here, too, is a fuller sense of Campbell's own personal drama, as he fended off depression and argued with Fiona Millar, his partner, who wanted him to quit.
Now we know that Blair had decided to sack Brown in January 2003, just when it was too late for him to do so. By then, war was upon us and Blair knew that the reshuffle would have to wait until after the invasion of Iraq. "It can only happen when the waters are calm and it's least expected, and it must be a totally ruthless operation," Blair told Campbell. But Iraq allowed no respite. Instead, it weakened Blair so much that he could not move Brown without risking his own position.
Blair pressed ahead, telling Campbell that it felt "like driving a car with the handbrake on". After every volume of these diaries, the reader is left with the question: "Why did Blair put up with Brown for so long?" This time the question is most insistent, and yet the answer is most convincing: that Blair would have brought his tenure of No 10 to a premature end if he had sacked him. For all the appalling personal behaviour – "He basically treats me like shit," Blair complains at one point – Brown was popular in the country and in the party.
It should not be forgotten that in 2005, Chancellor Brown was the best-rated politician in the opinion polls. Yet internally, the situation was one of irretrievable breakdown much earlier than the most fevered journalistic imagination could have envisaged. What is extraordinary is that Blair continued as prime minister for four years after this part of the story ends, and even managed to win another election, with Brown trapped by his side.
As ever, the immediacy of Campbell's account is engrossing. It is yet again a reminder of the salutary fact of history: that its actors do not know what is coming next. Thus Iraq features hardly at all in the first half of 2002, and surprisingly little after that, until Blair hits the crisis, with a month to go, of realising that he is unlikely to secure a second UN resolution explicitly to authorise military action.
And, as ever, the diaries' detail is telling. "TB said Ed Balls had brushed shoulders with him when he walked by, like kids in a schoolyard do." Some of the vignettes of Cabinet meetings are priceless: "TB started a sentence: 'If John Prescott had come to me in '97 and said renationalise the railways, it would have been a short conversation.' 'It was a short conversation,' JP interrupted."
Most important from a historian's point of view – apart from the further particulars of the Blair-Brown hatefest – is the further light on the decision to join the US invasion of Iraq. According to Campbell, there was a "real danger" in January 2003 that Lord Goldsmith, the Attorney General, "would resign if he thought the plan was disproportionate force". That was before Goldsmith went to the US for talks with the Americans and the British legal team at the UN, when he decided, in Campbell's words, that there was a "reasonable case for war" in international law, but also "a case to be made the other way".
Goldsmith was still, nine days before the military action started, saying that "he did not want TB to present" his legal advice "too positively". Within hours, though, Andrew Turnbull, the Cabinet Secretary, had "really irritated TB when he said he would need something to put round the Civil Service that what they were engaged in was legal. TB was clear we would do nothing that wasn't legal, and gave him a very heavy look." Goldsmith then wrote the short version of his advice, which left out the "case to be made the other way".
That – rather than the suggestion made by Jack Straw, the Foreign Secretary, the next day, that Britain should stand aside from the first wave of the invasion, which Blair would never have accepted – is the real "what if" of Britain's role in the Iraq war. What if the Attorney General had not been persuaded to certify that the war was legal?
Then, paradoxically, Blair would have been the hero of the peace movement in Britain and across Europe. And Brown might never have been Prime Minister.
John Rentoul is chief political commentator for The Independent on Sunday