A whitewash, a cover-up, "an establishment stitch-up", as David Cameron puts it: never has any investigation been so damned in advance as the Chilcot inquiry. And its composition did not suggest any lively zeal to expose the iniquity of the Iraq war, or the misdeeds of the last prime minister.

Himself the mandarins' mandarin, Sir John Chilcot was the very model of a modern permanent under-secretary, and then "staff counsellor" to MI6. Two of his team, Sir Lawrence Freedman and Sir Martin Gilbert, have been denounced as "Zionists" by Richard Ingrams, which may be less to the point than the fact that Freedman wrote speeches for Tony Blair, or that Gilbert, Winston Churchill's official biographer, had earlier compared Blair with Churchill. And the questioning of Sir John Scarlett, the former intelligence chief deeply complicit in the false prospectus for the war, was absurdly tame.

But you never can tell. Even before Blair appears in person, the inquiry has already been excruciating for him. Not only has such reputation for either honesty or judgement as he still possessed been shredded, while the ambassadors and generals settle the score. What has been less noticed is that, partly thanks to Chilcot and partly to other events, every single reason Blair ever entertained for the Iraq war has disintegrated. That means not only the arguments made to Parliament and people, but also his private convictions.

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By now we know not only that Saddam Hussein had no so-called weapons of mass destruction, but also that there was little real ground for saying that he did, something that witnesses at Chilcot have been rubbing in. Sir William Ehrman was director of intelligence at the Foreign Office in 2002-04, and says Downing Street was continually warned how limited "WMD" intelligence really was and that "Iraq might lack warheads capable of effective dispersal", the opposite of what Blair was then telling us.

But what of Blair's larger motives? He had become a great proponent of the doctrine of liberal or humanitarian intervention. Ten years ago, after the Chicago speech which preached that doctrine and the Kosovo operation which practised it, he enjoyed a blissful heyday, hailed by American liberals as "the prime minister of the United States", or even, in Paul Berman's words, "the leader of the free world".

That Chicago speech was a lucid exposition, no doubt because it was largely written by Freedman. "How do we decide when and whether to intervene?" Blair asked, and said that we must be "sure of our case ... have we exhausted all diplomatic options? ... are there military operations we can sensibly and prudently undertake? ... are we prepared for the long term? ... and finally, do we have national interests involved?" Since he asked, we can now ask in return: Were all those conditions fulfilled four years later in Iraq? Was any? And how does this look now?

Those who follow the doings and sentiments of Labour agree that, little as that bedraggled and demoralised party is now sure what it stands for, everyone acknowledges that liberal interventionism is dead and buried.

For a time Blair also persuaded his credulous followers that the invasion would revive the "peace process" and lead to a just settlement of the Israeli-Palestinian conflict. That has proved even more of a tragic illusion, which is one reason why watching Barack Obama in Oslo on Thursday was almost unbearable, quiet apart from the fact that a man who had just increased hostilities in a barely justifiable and very likely unwinnable war trying to explain why he should accept the Nobel Peace Prize.

What was even more dispiriting was to compare his flowery words with what an unnamed western ambassador in Israel was quoted as saying on the BBC World Service the other day: President Obama and Hillary Clinton had between them "driven the peace process into a ditch". The bleak truth is that a settlement is much further off today than 10 years ago.

Above all, Blair had one deep personal conviction, that it was his duty to support Washington. As he said just before the invasion, "It would be more damaging to long-term world peace and security if the Americans alone defeated Saddam Hussein." His mission was to embrace Bush so as to "keep the United States in the international system".

And so the most devastating evidence of all heard by Chilcot has described Blair's sheer servility when dealing with George Bush – and the way that he received nothing whatever in return. The British were deeply concerned about the legality of the war, and about post-war planning, or the lack of it. Washington paid no attention whatever.

As Sir Christopher Meyer, the ambassador in Washington at the time, told Chilcot, Blair again and again failed to get any get quid pro quo from Bush. Blair didn't even use such influence as he had when – this a sign of American gratitude for his support after 11 September – Congress imposed tariffs likely to destroy what was left of the British steel industry.

Worse still, any attempt by Blair's to revive the peace process "failed miserably", in Meyer's brutal words: "We could have achieved more by playing a tougher role." Blair could have told Bush he would not commit British troops "unless we have palpable progress on the peace process". And in the unkindest cut of all, Meyer added, "I think Margaret Thatcher would have ... insisted on a coherent political and diplomatic strategy."

While Tony Blair sees his reputation being trashed daily, he may dimly perceive that the interventionist doctrine he once espoused is utterly discredited. As he sits in Jerusalem on his futile mission, he can see, if he bothers to look, that the peace process is dead. When he visits JP Morgan's Wall Street office to collect his sinecure, or when – more bizarrely – he lectures on ethics at Yale, he might recall what Gerhard Schröder said about "the 'special relationship' so special that only the English know it exists", a truth Blair has dramatically personified.

A well-worn line says that all political careers end in failure. Has any ever ended in such failure as this?

Geoffrey Wheatcroft's books include Yo, Blair!, published by Politico's Publishing