He was the most plodding, the most robotic, and – until this week – apparently the most loyal of presidential spokesmen. But now Scott McClellan, White House press secretary for George Bush between 2003 and 2006, has delivered the most wounding critique yet of this unhappy administration by one of its erstwhile senior officials.

What Happened: Inside the Bush White House and Washington's Culture of Deception is no falsely touted insider memoir, jazzed up with a few titillating anecdotes to boost sales. It is a 341-page disquisition on Mr Bush, on his misbegotten war in Iraq, and on his entire conduct of the presidency, which Mr McClellan says was built on the use of propaganda, and on the technique of government as permanent campaign.

"History appears poised to confirm," he writes in arguably the most damning paragraph of a book full of them, "that the decision to invade Iraq was a serious strategic blunder. No one, including me, can know with absolute certainty how the war will be viewed decades from now ... What I do know is that war should only be waged when necessary, and the Iraq war was not necessary."

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And those are not the words of a disgruntled outsider, summoned to the colours and then casually tossed aside. Mr McClellan largely owes his career to Mr Bush. He was spokesman for Mr Bush and part of the "Texas Mafia" along with the likes of Karl Rove and Karen Hughes.

A man with deep political connections in the Texan capital, Austin, Mr McClellan first worked for then governor Bush in early 1999. He was travelling press secretary for the Bush-Cheney campaign in 2000 before becoming chief deputy White House spokesman in the first Bush term. In July 2003, he took over from Ari Fleischer, and served as press secretary for almost three years.

It was a wretched period. True, his boss did win a narrow re-election in 2004 but, thereafter, it was downhill all the way. The draining CIA leak affair (in which Mr McClellan claims he was misled by both Mr Rove, Mr Bush's closest adviser, and by Lewis "Scooter" Libby, Vice-President Dick Cheney's chief of staff who was ultimately convicted of perjury and obstruction of justice) was followed by Hurricane Katrina and the administration's disastrously botched response, and by ever growing public disenchantment with the war. By the time Mr McClellan was eased out in April 2006, Mr Bush had become one of the most unpopular US presidents of recent times, and has remained so ever since.

In its own words, What Happened is a chronicle of "how the presidency of George W Bush veered terribly off course". Its longer term impact may be limited, by dint of the fact that the Bush presidency has sunk so low that it can hardly fall further. Mr McClellan's "revelations" moreover merely confirm what all but the most blinkered supporters of the 43rd President have long since realised. But the immediate reaction of the Bush camp has been predictably bitter. Officially, the White House brushes off the book. Unofficially however, the President's men are vitriolic, claiming he did not know what was going on but has turned upon his former boss to boost his book royalties.

"It shows how out of the loop he was," Mr Rove, the man once known as "Bush's Brain", said on Fox News where he is now a commentator. "This doesn't sound like Scott, it sounds like a left-wing blogger. I don't remember him speaking up [about the concerns laid out in the book] at the time."

In fact, Mr McClellan's portrait of the President – a man he says he still respects and admires – is far more nuanced. Which of course only makes it more telling. Mr Bush comes across in now familiar guise, as a skilled politician, possessed of charm and an engaging wit, who is, "plenty smart enough to be President". On the other hand, he is utterly incurious and uninquisitive on policy matters, preferring to rely on gut instinct than a detailed sifting of the arguments.

For the 43rd President, a decision once taken is always right. The approach reflects not only Mr Bush's ingrained stubbornness but his ability to deceive not only others, but also himself. Mr McClellan offers as illustration a moment on the campaign trail in 1999, when he heard the governor/candidate talking on the phone to a friend about reports that he had used cocaine in his youth. Apparently, Mr Bush remarked that ... "the media won't let go of these ridiculous cocaine rumours. The truth is I honestly don't remember whether I tried it or not. We had some pretty wild parties back then, and I just don't remember."

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In 2000 voters – battle-hardened by having to confront Bill Clinton's marijuana use ("I did not inhale") and explain to their curious children the finer points of the Monica Lewinsky affair – did not seem greatly bothered. They assumed Mr Bush might indeed have indulged in cocaine, just as he had indulged in the bottle which he had emphatically given up. But Mr McClellan drew a different lesson from the episode. "I remember thinking to myself, how can that be?" he writes. "How can someone simply not remember whether or not they used an illegal substance like cocaine? It didn't make a lot of sense."

On the other hand, Mr Bush wasn't, "the kind of person to flat-out lie." So, Mc McClellan concludes, "I think he meant what he said in that conversation about cocaine ... I felt I was witnessing Bush convincing himself to believe something that was not true, and that, deep down, he knew was not true. And his reason for doing so is fairly obvious – political convenience." And thus, by implication at least, it was with Iraq and Saddam Hussein's non-existent weapons of mass destruction.

What Happened may throw new light on the enduring mystery of the war: why exactly did Mr Bush decide to invade a country that even he knew had nothing to do with the 9/11 attacks that triggered his "war on terror?"

In a 2003 interview with Vanity Fair, Paul Wolfowitz, then Deputy Secretary of Defence and intellectual architect of the war, gave a hint when he suggested that WMD were only one reason for the invasion – "something everyone could agree on". Mr McClellan goes significantly further. The administration's real motive for war, he declares, was the neoconservative dream of creating a democratic Iraq that would pave the way for an enduring peace in the region.

But the White House had to sell the war as necessary because of the threat posed by Saddam Hussein. They accordingly took a different tack, not of "out-and-out deception", but of "shading the truth". This was achieved by "innuendo and implication", and by "intentionally ignoring intelligence to the contrary".

But, one might ask, what else is new? An identical conclusion after all was reached as early as the summer of 2002, in the celebrated Downing Street memo in which British officials just back from a visit to Washington said US intelligence was being shaped to fit a decision to go to war .

It is, however, astounding to hear this critique from the man who spent the best part of three years doggedly defending the war and its consequences from a press corps that (as he writes in the book) had given the administration far too easy a ride in the run-up to the war – and was bent on making up for that omission when Mr McClellan succeeded Ari Fleischer as press secretary in summer 2003, when no WMD had been found, and it was all but certain none would be.

Even more astounding is his assertion that, contrary to everything the President continues to insist (aided no doubt by that talent for self-deception) Mr Bush would take his war back if he could. "I know the President pretty well," Mr McClellan writes. "If he had been given a crystal ball in which he could have foreseen the cost of war, more than 4,000 American troops killed, 30,000 injured, and tens of thousands of innocent Iraqis dead, he would never have made the decision to invade, whatever he says or feels he has to say publicly today."

Blame does not belong with Mr Bush alone. What Happened delivers tough criticism of the President's once vaunted national security team. One member of it of course was Dick Cheney, referred to by Mr McClellan as "the magic man" who somehow "always seemed to get his way" on every issue that mattered to him, be it the war, boosting the executive power of the presidency, or the harsh treatment of detainees.

Even more damning is his verdict on Condoleezza Rice, national security adviser in the run-up to the invasion. Her main talent, Mr McClellan suggests, was a Teflon quality. Whatever went wrong, "she was somehow able to keep her hands clean," even when the problems related to areas for which she was responsible, such as the WMD rationale for war (including the infamous "16 words" in the 2003 State of the Union address about Saddam seeking uranium in Africa, that led to the CIA/Valerie Plame affair) and the planning for post-war occupation. History, he predicts, will not be kind to Ms Rice. But "she knew well how to adapt to potential trouble, dismiss brooding problems and always come out looking like a star".

That is more than could be said for Mr McClellan himself, with his consistently gloomy demeanour and lack of the eloquence or sense of humour required to extricate himself from tight corners in the press room.

Rarely did he come out looking like a star. Equally rarely however did he look like a man secretly thirsting for revenge, even when he was replaced in spring 2006 by the conservative broadcaster Tony Snow (who, whatever else, was never lost for words).

Today Mr McClellan has found his words, in print. He professes still to like and admire his old boss. To which Mr Bush can only conclude, with friends like this, who needs enemies?