The Chilcot inquiry today met its first "hostile" witness, Sir John Scarlett, former head of MI6. Mastermind of Saddam's threat of weapons of mass destruction and thus architect of Tony Blair's case for the Iraq war, he entered the inquisition room like a small, well-bred bull, ready for battle. Within seconds he was wandering round the ring, lost and searching for a matador. The inquiry appeared to have gone on strike.

Scarlett duly droned for a third of his allotted time on the structure of the joint intelligence committee.

The inquiry members looked to the ceiling, gazed at their feet, even seemed to fall asleep. Scarlett teased them with tales of dossiers and spin, with murmurs of American pressure, aluminium tubes and the clear impression that weapons inspectors were spies. They barely noticed. He failed to mention Alastair Campbell or Tony Blair. He did all he could to cause a fight, but he failed. He walked out unmarked. Chilcot is an inquiry with much to prove.

For two weeks, the investigation into the alleged failures of the 2003 invasion of Iraq has dealt with processes and procedures. One elegant mandarin after another has paraded, well-rehearsed, before it. Rarely do more than a few onlookers grace the airless room, overwhelmed by infantile government security. At one session a group of bemused tourists declared it "at least better than the House of Lords". This may be merely a prologue to the star turn, Blair, who is not due until next year. But Scarlett was the star's apprentice, and the place was for once packed and expectant.

When pressed on being told to "firm up" the intelligence of weapons of mass destruction in 2002, Scarlett was left to declare blandly that that is what he did. When asked if there was any coercion from America, he said no.

When asked if perhaps the September dossier, and its 45-minutes warning, was confusing, he said probably. When asked if he might have disapproved of Blair's "without doubt" interpretation of it, he said maybe.

I never thought I would cry "send for a lawyer" but the inquiry desperately lacks a skilled cross-examiner, someone who at least knows the word supplementary. The inquiry's two historians, Sir Martin Gilbert and Sir Laurence Freedman, appear to be researching their next book. Lady Prashar is interested only in "clearing things up". The diplomat Sir Roderic Lyne occasionally leaps to inquisitorial life, but not when faced by the head of MI6. This was like a private conversation in a Whitehall club.

For all that, a picture is starting to emerge from Chilcot. It is of 2002 and an ever more lonely Blair, desperate to be "a serious player" on the world stage. He is trapped between what his Washington ambassador, Sir Christopher Meyer, eulogised as his "enormously close relationship" with George Bush, and British lawyers telling him an invasion would be illegal, British generals saying an occupation would be a shambles and cabinet colleagues thinking him mad. (I hope we hear from some of them.)

The inquiry so far has been dominated by two themes, the chaos of the American occupation of Baghdad, and the zeal of the Foreign Office to drive a stake through Blair's heart at the nearest crossroads, for destroying Britain's reputation in the diplomats' beloved Middle East. Rarely can Whitehall's finest have turned so savagely on a recent boss. The FCO's chief, Sir Peter Ricketts, was blunt: "We quite clearly distanced ourselves from talk about regime change," which Blair had mooted as early as 1998. His colleague, Sir William Patey, said that when Bush came to power, "we heard the drumbeats from Washington … and our policy was to stay away from that part of the spectrum. It had no basis in law." The illegality of the invasion is a leitmotif, yielding Chilcot's one inadvertent scoop, a leak of a letter submitted by the then attorney-general, Lord Goldsmith, to Blair in 2002. This declared that the invasion had "no legal basis for military action … as things stand you obviously cannot do it." When Blair ignored the letter and banned Goldsmith from cabinet, the attorney general reportedly threatened to resign and famously lost three stone in weight. Just two weeks before the invasion, Goldsmith was still warning the cabinet, as well as the chief of the defence staff, Admiral Lord Boyce, that British soldiers could be "arraigned before the international criminal court" if they went to war. This led Boyce to demand "unequivocal advice" that the war was legal. Goldsmith duly changed his mind. The then lord chancellor, Lord Falconer, has publicly dismissed the spin put on the letter as "totally false". Since he and Goldsmith cannot both be right, their cross-examination in the new year should be the next test of Chilcot's muscle. They should be forced to appear together.

The spilling of Blair's blood so far has come not from the inquiry but from the witnesses. Bush is portrayed as treating Blair as a patsy. One official after another has rubbished Blair's claim that intelligence indicated "beyond doubt" that Saddam had mass destruction weapons and intended to use them. Since the phrase was not Scarlett's, the finger points to Blair's Downing Street team. It remains to be seen how many will be called to give evidence. The Butler report on WMD intelligence omitted all mention of Blair's spin doctors.

The chief respite for Blair has come from his foreign policy aide, Sir David Manning, and from Meyer. The former offered a model display of graded loyalty to his boss. Manning stressed Blair's commitment to "the UN route", to giving the weapons inspectors enough time and to ensuring cover for public opinion back home. This contrasted with Meyer's evidence, that Blair had been gung-ho for regime change since 1998 and that his bond with Bush at the Crawford meeting in April 2002 was probably "signed in blood".

Blair's lack of influence in Washington is becoming ever more stark. Only the possibility that he might lose a Commons vote on going to war seems to have moved Bush to attempt another UN resolution. As the aid department's Sir Suma Chakrabarti said yesterday, he and his colleague could not believe America's lack of concern for the UN, indeed for world opinion, believing that "rationality would break out at some stage". It did not. The Americans did not care what their allies did or did not do. It was Blair who seemed desperate, according to the deputy chief of the defence staff, Sir Anthony Piggott, to do "something meaty on the ground".

Blair's eagerness seems to have cost Britain all leverage. Meyer was forced by Lyne to confront the central question, whether Blair could have avoided going to Iraq without damage to British interests. Meyer's answer was yes. Bush even phoned Blair to suggest he could "sit out the war", while the Pentagon's Donald Rumsfeld was happy to go in alone. But Blair wanted too much to be there. So far, said Meyer, "we had underestimated the leverage at our disposal". Now it evaporated.

Meyer has been the undoubted star of the show so far. In a startling but unnoticed revelation, he mentioned that Blair refused even to use his good offices with Bush to lobby for relief from tariffs on Britain's special steel or seek domestic slots for Sir Richard Branson's Virgin planes. Blair was hugely popular but his clout in Washington was exhausted. Thanks to him the pre-Iraq phase was an awful episode in British diplomacy. No wonder the Foreign Office wants history to free it of blame.

More serious was the frustration clearly faced by the army. Admiral Lord Boyce told the inquiry that he was banned by the defence secretary, Geoff Hoon, from actively preparing for invasion since it might suggest Britain was not serious about seeking the abortive UN resolution. In the understatement of the inquiry, Boyce said he found this ban, just months from a putative invasion, "very frustrating". He could not even talk to his own head of logistics. Boyce added that he found the whole American approach "anorexic", largely because of "disfunctionality" between departments in Washington. He himself had sometimes to act as go-between. This led to the Americans being desperately understaffed on the ground when trouble began in late-2003. While the lack of post-invasion planning is hardly news – there is a shelf of memoirs on it – Whitehall's desperation to put its warning of chaos on the Chilcot record is palpable.

The FCO's Iraq expert, Edward Chaplin, spoke of neo-con Washington's "real blind spot", indeed its "touching faith", that there would be "dancing in the streets after the invasion … all sweetness and light". Major General Tim Cross, stationed in Baghdad, said he told Blair that post-war planning was "chaotic", but Blair just stared. On his arrival in the city after the invasion, Cross told of his "amazement" at the shambles that greeted him. Entire government departments were being run from single tables in Saddam's palace corridor, those in charge changing by the week.

The purpose of this inquiry remains obscure. Its tales are familiar to those who have followed the war, and such interest as exists comes largely from hearing the old tales from the horses' mouths. Sir John Chilcot treats witnesses like a therapist with a nervous patient. The absence, at least so far, of any Iraqis, Americans, foreigners of any sort or even British politicians has become glaring. If this is to be a first rough draft of history, it is so far a highly partial one.

Chilcot emphatically rejects being cast as a court, let alone a foretaste of a Nuremburg trial. It is a far cry from the scrutiny of America's Capitol Hill or the milder forensic thrust of a Hutton or a Butler. This appears as a very British inquest, an intrusion into the private grief, or perhaps the self-styled triumph, of one man, Tony Blair.

But who knows? Perhaps still waters yet run deep.

