The Chilcot inquiry summoned Tony Blair last week to establish facts for the historical record. But Mr Blair turned up to win history's approval, its gratitude even. The result was pure theatre; the fluent former prime minister was given the stage and the cues. He performed.

The questioning was neither forensic nor consistent enough to disturb Mr Blair's composure, let alone force him into embarrassing disclosure. It was certainly insufficient to satisfy those of his critics – the most vocal if not the most numerous constituency – who would have preferred something more like a criminal interrogation.

But the Chilcot inquiry is not a court.

There was no perfect solution to Britain's Iraqi dilemmas in 2002: questions over the legality of invasion; the threat of weapons of mass destruction; the nature of a duty of loyalty to the US. But decisions had to be taken. Mr Blair insists he made choices in good faith. The vital question is not whether he is lying, but whether his judgments were the right ones.

He was not alone in trying to navigate conflicting arguments and incomplete evidence. The nation was divided. This newspaper supported a war that, with hindsight, it should have opposed.

For many, it was the defining choice of Mr Blair's premiership. It is hardly surprising, then, that he still stands by it. His defence relies on the assertion that the course he took, albeit painful and bloody, was right because it resulted in the removal of Saddam Hussein. The means, in other words, were justified by the end.

But were they? No rational critic of the war opposed Mr Blair out of affection for Saddam. It was not the desire that Iraq should be governed differently that caused problems, but the insistence that change be effected immediately and by force.

In that respect, the inquiry failed to clarify a crucial point: the difference between Mr Blair's private commitments to President Bush in support of war in 2002 and his public claim, up until the eve of hostilities in 2003, that Britain wanted a diplomatic solution.

The US believed Saddam would never comply with weapons inspectors. Washington was bent on regime change.

Having volunteered Britain for that project, Mr Blair then tried to steer it through the United Nations. That meant presenting to the world, in the form of UN resolutions, an opportunity for Iraq to avoid invasion. But it was a fiction. The date for war was set.

Claims about Iraqi WMD must be seen in that context. Mr Blair says he trusted the intelligence reports, later made public in the now infamous "dossiers". There is no reason to doubt the sincerity of his belief that Saddam had a WMD programme and that it might, at some stage, be used in collaboration with terrorists against the west.

But by his own admission, Mr Blair did not think the threat imminent. It was purely hypothetical. The important fact was that, after the 9/11 terrorist attacks, he decided even hypothetical risk was intolerable.

That is not how the case was presented in public. Mr Blair would never have won Parliament's or the public's support for a war to snuff out a small, niggling threat.

In a most disingenuous passage of testimony, Mr Blair said he ought to have corrected some exaggerated media claims about the WMD threat, but paid them little heed at the time. Nonsense. Downing Street had powerful machinery for influencing public opinion. It was set full throttle to win support for war.

Therein lies a source of anger that Mr Blair cannot grasp. The offence was not believing faulty intelligence, it was the tendentious presentation of information to secure a political objective, as if the act of sending soldiers to invade another country could be managed like some public sector initiative.

Mr Blair treated law and constitutional propriety as barriers to the expedient pursuit of his goal. He overcame them by force of will. It was effective, but it was not democracy.

And what of the end that should have justified those means?

In his testimony, Mr Blair painted a picture of Iraq emerging blinking and frail from the darkness of despotism into the sunlight of freedom. Most Iraqis, he argued, are better off now.

That is a rhetorical trick. There is little doubt Iraq is generally glad to be rid of Saddam. But that does not mean everything that happened in the country after March 2003 was necessary.

Much of the bloodshed was a consequence of US failure to plan for postwar stabilisation. The Bush administration was scornful of nation-building and cavalier in its under-estimation of troop requirements. The peace was lost before the war was even won.

If Mr Blair did not challenge Mr Bush on his lack of preparation, he shares blame for the deadly consequences. If he did warn and was rebuffed, he must accept the charge that Britain's support for the war earned it no greater diplomatic weight in Washington. Either way, British power failed.

Mr Blair told the inquiry it was in British interests to go along with every aspect of American policy. But he did not say how. The disadvantages are plain. The UK's status on the Security Council and its influence in the Muslim world are diminished. Britain is more of a target for terrorist attack. Its armed forces were overstretched and, in their messy retreat from Basra, humiliated.

The Iraq war made the Middle East less secure. It emboldened Iran and made future moves to disarm Tehran doubly difficult. It debased the moral case for humanitarian intervention. It undermined the authority of international law, legitimising unilateral action by bullying governments.

Mr Blair would have us consider a narrative of alternative history that makes it all worthwhile: Saddam was left in power, he used a failing sanctions regime to develop WMD and passed them to al-Qaida. An atrocity worse than 9/11 followed.

But that dark fantasy assumes there were no middle ways, that every choice Mr Blair made was the best one available. It is easy to see why Mr Blair should want to believe that is the case, but it isn't.

Mr Blair believes the end justified the means. But the methods used to take Britain to war perverted law and democracy. That was not some unfortunate byproduct of a greater moral endeavour, it was a fatal corruption of the diplomatic process that might have led to a better outcome for Iraq. The means sabotaged the end.

Mr Blair insisted last week that the issue at stake is not "a lie, a conspiracy or a deceit" but "a decision". Fair enough. Ultimately, the Chilcot inquiry should be measuring his political judgment. He alone made the choice to take Britain to war. He was wrong.