Tony Blair is damned. We have seen establishment whitewashes in the past: from Bloody Sunday to Hillsborough, officialdom has repeatedly conspired to smother truth in the interests of the powerful. But not this time. The Chilcot inquiry was becoming a satirical byword for taking farcically long to execute a task; but Sir John will surely go down in history for delivering the most comprehensively devastating verdict on any modern prime minister.

Those of us who marched against the Iraq calamity can feel no vindication, only misery that we failed to prevent a disaster that robbed hundreds of thousands of lives – those of 179 British soldiers among them – and which injured, traumatised and displaced millions of people: a disaster that bred extremism on a catastrophic scale.

One legacy of Chilcot should be to encourage us to be bolder in challenging authority, in being sceptical of official claims, in standing firm against an aggressive agenda spun by the media. Lessons must be learned, the war’s supporters will now declare. Don’t let them get away with it. The lessons were obvious to many of us before the bombs started falling.

For what Chilcot has done is illustrate that assertions from the anti-war movement were not conspiracy theories, or far-fetched, wild-eyed claims. “Increasingly, we appear to have a government who are looking for a pretext for war rather than its avoidance,” declared the anti-war Labour MP Alan Simpson weeks before the invasion. And indeed, as Chilcot revealed, Blair had told George W Bush in July 2002: “I will be with you, whatever.”

This, as Chilcot puts it, was no war of “last resort”: this was a war of choice, unleashed “before the peaceful options for disarmament had been exhausted”. Simpson said: “We appear to produce dossiers of mass deception, whose claims are dismissed as risible almost as soon as they are released.” And now Chilcot agrees that the war was indeed based on “flawed intelligence and assessments” that were not “challenged, and they should have been”. Nelson Mandela was among those who, in the runup to war, accused Blair and Bush of undermining the United Nations. Mandela lies vindicated. As Chilcot says: “We consider that the UK was … undermining the security council’s authority.”

So many warnings. A month before the invasion the US senator Gary Hart said that war would increase the risk of terrorism. “We’re going to kick open a hornet’s nest, and we are not prepared in this country,” he warned.

Consider this, from the anti-war Dissident Voice website a month before the conflict: “A US attack and subsequent occupation of Iraq will provide new inspiration – and new recruitment fodder – for al-Qaida or other terrorist groups, and will stimulate a long-term increased risk of terrorism, either on US soil or against US citizens overseas.” It is not to belittle the authors to point out this was a statement of the obvious, except to those responsible for the war and their cheerleaders. Then read Chilcot: “Blair was warned that an invasion would increase the terror threat by al-Qaida and other groups.”

The former prime minister claimed that the terrible aftermath was only obvious in hindsight, yet Christian Aid warned of “significant chaos and suffering in Iraq long after military strikes have ended”. An aid agency had far better foresight than the senior general who – at an off-the-record chat I attended at university – claimed that 99% of Iraq would be throwing flowers at the invading soldiers. As Chilcot put it, the government “failed to take account of the magnitude of the task of stabilising, administering and reconstructing Iraq”.

Blair’s risible claim is wrong: as Chilcot puts it, “the conclusions reached by Blair after the invasion did not require the benefit of hindsight”. The threats of everything from Iranian meddling to al-Qaida activity “were each explicitly identified before the invasion”.When Robin Cook resigned from the cabinet before the invasion, he declared that “Iraq probably has no weapons of mass destruction in the commonly understood sense of the term”. Chilcot has now damned the intelligence services for believing otherwise.

The Campaign for Nuclear Disarmament threatened a legal challenge against the government in 2002 if it went to war without a second security council resolution. Several lawyers and Kofi Annan, the then UN secretary general, are among those who have since described the invasion as illegal.

The original advice by the UK attorney general, Lord Goldsmith, was indeed that a war without a second resolution would be illegal, but Chilcot highlights the fact that by the time Goldsmith gave a subsequent oral statement he appears, mysteriously, to have changed his mind.

The legality of the war may not have been within Chilcot’s remit. But even then he finds that the process through which the government arrived at its legal basis “was not satisfactory”. Surely the legality of this calamitous war must now be challenged in a court of law.

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We always claimed that the Iraq war was based on lies. Reading prewar articles, such as “The lies we are told about Iraq” in the Los Angeles Times, is instructive indeed. The Chilcot report did not accuse Blair of lying. But too much emphasis is put on this question. Blair was clearly determined to go to war long in advance. He relied on dubious evidence to make his case, evidence that others at the time knew to be dubious. Did he deceive himself, or the public, or was he just driven by the righteousness of a messiah complex? He pursued a war with a dodgy prospectus that many at the time – including 139 Labour MPs – knew would result in disaster. And that is damning enough.

Let’s laud the Chilcot inquiry for giving the official seal to the truths we have always known, but be aware that this is all it has done. The truths it has exposed were there already, long before the gates of hell had been opened – as the secretary general of the Arab League warned would happen, before the invasion.

It was the obviousness of what was going to happen that created the biggest anti-war movement in history. It was a movement belittled, not least by media that largely backed the rush to war. How perverse it was that those who opposed or criticised the war – from politicians to the BBC’s bosses – were the ones to lose their jobs, while Blair has since pursued his lucrative career working for dictators.

Many cheerleaders of this great catastrophe still show scant remorse or penitence. Some even heckled the Labour leader, Jeremy Corbyn – who campaigned against both Britain’s backing for Saddam Hussein when he gassed the Kurds in the 1980s, and the 2003 invasion – as he delivered his parliamentary response to Chilcot today.

And the horror continues, the 250 Iraqis killed by car bombings this weekend a devastating reminder of the chaos for which Blair must take responsibility. This was not a blunder, not an error, not a mistake: whatever the law decides, this was – from any moral standpoint – one of the gravest crimes of our time. Those responsible will be for ever damned. After today, we can single them out – and call them by name.