There was a moment during Tony Blair's appearance before the Iraq inquiry – it came in the stretch when his limp inquisitors were fumbling an attempt to pin him down on the legality of the war – that the former prime minister gave them a knowing smirk and remarked: "I was a lawyer myself. I wrote many opinions." As if we really needed any reminding.

He occupied Number 10 for 10 years and 55 days. Tony Blair did not sit at the pinnacle of power for a double-digit stretch without having a first-class capacity to master any brief, however dodgy, mount any argument, however specious, slide past any question, however difficult, parry any critique, however piercing, and defend any case, however indefensible. The Iraq inquiry puts his historical legacy in the dock. So he had hired the slickest lawyer he could find. That would be himself.

He was also one of the greatest actor-managers to sit in Downing Street. When he first sat down before the panel, clutching his lever arch file of supporting documents like a comfort blanket, he seemed to be suffering a touch of stage fright about the prospect of returning to the theatre of British politics for the first time in nearly three years. His face looked taut and he opened a bottle of water with a shaky hand. Yet he was soon at ease, relaxing into the performance, almost beginning to enjoy this enforced comeback, remembering all his old lines about Iraq and delivering a few fresh ones about Iran which he had scripted to command the attention of that night's news bulletins and the next morning's front pages. He had come with a plan, which he executed successfully, to divert an inquiry into what he did in 2003 into a lecture about world affairs and the demands of leadership in 2010.

It was a six-hour reminder that he was – and remains – the consummate political performer of his era. That was one of the reasons he managed to persuade the rest of a Labour government, against all its natural instincts, to follow him into a hugely contentious war alongside a very right-wing American president. He also won a hat trick of elections, securing the third victory even when the war had gone horribly wrong and its prospectus was exposed as bogus.

So Sir John Chilcot's panel would have to be very good indeed to lay a glove on a man who has already ducked and weaved his way through seven years of questions – by previous inquiries, in Parliament, by the press, on television and radio and in his own head. Very good indeed Chilcot and co are not. I did not expect this to be Tony Blair "on trial", but I had hoped that his inquisitors would display a bit more cutting edge than a month-old lettuce. Time and again, they approached an interesting subject area, stumbled around like people in the dark trying to find the light switch and then abandoned the quest without leaving themselves or anyone watching much the wiser about the most divisive war in the last century of our history.

Some say they are so feeble because four knights and a dame are establishment worthies who are temperamentally incapable of speaking truth to power – or even asking power some awkward questions.

That, I think, is a little unfair. The former civil servant, ex-ambassador, quango queen and two historians are clearly troubled by some of the evidence. Sir Roderic Lyne, once our man in Moscow, has a nice line in dry incredulity.

No, the real problem with this panel is that they are simply not up to it. They are incapable of constructing a line of questioning with enough forensic focus to pin down the truth. This is a disability especially obvious when they are dealing with such an accomplished operator as Tony Blair, a man with years of experience of evading far more artful and rigorous inquisitors than these. I have seen trainee television researchers come up with better constructed interviews than the interrogation of the former prime minister.

This is a crying shame. We already know a lot about why Britain joined the war and what went so grievously wrong after the fall of Saddam, but I don't agree with those who contend that we know so much that further inquiry is unnecessary. Essential details are still in dispute and crucial events remain swirled in murk. The supposed task of the Iraq inquiry is "to identify lessons that can be learnt". That can't be done until we know precisely what those lessons are.

I am not coming at this as someone who ever expected a "Gotcha!" moment. There was never a chance that the former prime minister was going to break down in a blubbering confession to atrocious errors before handing himself over to the protesters outside who had mocked up a jail for him. Nor do I think that the only useful commentary on him is to yell "war criminal".

I appreciate the dilemma he faced once he knew that George W Bush was intent on toppling Saddam Hussein. I think he was utterly sincere in his fear of the atrocities that might be perpetrated as the result of a conjunction between rogue states, terrorists and weapons of mass destruction. I understand why he felt that it was a strategic imperative to stay in lock step with the United States and have never doubted that he had a genuine revulsion of Saddam Hussein. It was the combination of those two impulses that made him not a poodle to the White House but an enthusiastic collaborator in the project to remove the Iraqi dictator.

I am instinctively a liberal interventionist who thought that Tony Blair played a creditable role when British forces saved Sierra Leone from sadistic thugs and did so again when Slobodan Milosevic was stopped from ethnic cleansing in Kosovo.

One of the many tragedies of the Iraq war is that it will be hugely more difficult for any future British leader to persuade his country that there are times when it is not just right but an obligation to intervene when tyrannical states threaten their neighbours or their own people.

As any historian of this government or journalist of this era should be, I was green with envy that Sir John and his panel were granted such an invaluable opportunity to tease out some truths from Tony Blair. By the end, I was closer to being red with rage that they had squandered nearly all of their six hours. At each stage of the chronology and on every issue of contention, the key questions were pursued lamely or not at all.

We heard Tony Blair deny that he "signed in blood" with George W Bush at the Crawford summit in Easter 2002. Yet we already know, from memos leaked long ago, that both his ambassador in Washington and his senior adviser on foreign policy explicitly told the Americans a year before the war that Tony Blair backed "regime change".

We further know, thanks to a declassified document released by Downing Street on Friday evening which the panel had already seen, that Tony Blair was offering support to remove Saddam "when the circumstances were right" even before 9/11. The inquiry failed to grasp the opportunity to find out precisely what he signed up to during the many private hours he spent with George Bush at his Prairie Chapel Ranch in Texas. As a result, they never identified the moment – an absolutely crucial question – when he concluded that he would join the war.

The invasion was launched on a misleading prospectus constructed from intelligence which was flaky when it was not simply fake. We heard the former prime minister lightly dismiss this by saying that the infamously wrong dossier about WMD had "assumed a vastly exaggerated importance later".

Yet it was Mr Blair – as the panel did not remind him – who invested that document with such importance by recalling Parliament for an emergency session so that he could fanfare the dossier to MPs and the nation. He had been told by his own officials that the intelligence was "sporadic and patchy" and yet he represented it to the Commons as "detailed and authoritative". Asked why he had not asked essential questions about the nature of the so-called intelligence, he was allowed to escape with the insouciant shrug: "I didn't focus on it a great deal."

It is very hard to reconcile that with his contention that he was nevertheless convinced that it demonstrated a threat from Iraq that was "beyond doubt".

When it came to what he said to the attorney-general about the legality of the war, he fell back on Bill Clinton's favourite device of not being able to recall "any specific conversations". The panel never roused themselves to try to prod his memory into a better account.

He was "shocked and angry" by the abuse of Iraqi detainees at Abu Ghraib. His inquisitors didn't trouble to probe the extent of his government's knowledge of and potential complicity with the torture and other appalling acts perpetrated in the name of the alliance.

There was no credible explanation for why the postwar planning was so calamitously inadequate. Nor for why, when Iraq began to descend into murderous mayhem, he was incapable of getting George Bush to do anything about it. Some illuminating answers about that would come from American witnesses – I've talked to several of them – and one of the weaknesses of this inquiry is that it has not taken evidence from the other side of the Atlantic.

His most startling admission came towards the end and it tumbled out almost inadvertently. Tony Blair had not, he confessed, realised that al-Qaida would see the allied presence in Iraq as an opportunity to wage jihad against American and British troops. Nor had he foreseen that Iran would seize the chance to foment trouble for the allies. That was a remarkable confession of failure to anticipate what ought to have been utterly predictable.

When it came to Iraq, Tony Blair was a combination of passionate conviction and cavalier disregard for detail. It was that fatal marriage which explains so much of what went so wrong.

What did we really learn? We learnt that the former prime minister still thinks it was right to get rid of Saddam, is sorry about the odd mistake and would nevertheless do it all over again, given the chance. This leaves us, as a country, in a deeply unsatisfactory place about the Iraq war. There is no "closure" for those bereaved by the conflict and any expression of sympathy for them was astonishingly lacking from Tony Blair even when the chairman invited him to express any regrets.

The country as a whole is still denied a full account of the conflict that cost so much in both blood and treasure. For the government, there is no drawing of a political line under the war. It haunts them still, not an outcome to be desired by Gordon Brown when he is weeks away from a general election at which he needs every Labour voter he can get.

It is not a positive result even for Tony Blair. I guess he can look upon his testimony as a chance to road-test the defiant passages planned for the self-justificatory chapters of his memoirs. His friends say he feels comfortable with his performance and has no regrets about being so unapologetic because contrition would have handed ammunition to his enemies.

Yet there is a difference between getting away with it and settling the account. The very fluency of his performance, the practised ease with which he ran rings round a bunch of wet inquisitors who never once penetrated his defences, leaves a bad taste in the mouth and a continuing hunger to get at the whole truth.

This was certainly not Judgment Day. Tony Blair once said that he expected to have to answer to his Maker. Assuming they ever meet, perhaps he is right. Perhaps it is now beyond any earthly power to get a final reckoning from him for Iraq.