Little is more corrosive of democracy than impunity. When politicians do terrible things and suffer no consequences, people lose trust in both politics and justice. They see them, correctly, as instruments deployed by the strong against the weak.

Since the first world war, no British prime minister has done anything as terrible as Tony Blair’s invasion of Iraq. This unprovoked war caused the deaths of hundreds of thousands of people and the mutilation of hundreds of thousands more. It flung the whole region into chaos, which has been skillfully exploited by terror groups. Today, three million people in Iraq are internally displaced, and an estimated 10 million need humanitarian assistance.

Yet Blair, the co-author of these crimes, whose lethal combination of appalling judgment and tremendous powers of persuasion made the Iraq war possible, saunters the world, picking up prizes and massive fees, regally granting interviews, cloaked in a forcefield of denial and legal impunity. If this is what politics looks like, is it any wonder that so many people have given up on it?

The crucial issue – the legality of the war – was, of course, beyond Sir John Chilcot’s remit. A government whose members were complicit in the matter under investigation (Gordon Brown financed and supported the Iraq war) defined his terms of reference. This is a fundamental flaw in the way inquiries are established in this country: it’s as if a defendant in a criminal case were able to appoint his own judge, choose the charge on which he is to be tried and have the hearing conducted in his own home.

But if Brown imagined Chilcot would give the authors of the war an easy ride, he could not have been more wrong. The Chilcot report, much fiercer than almost anyone anticipated, rips down almost every claim the Labour government made about the invasion and its aftermath. Two weeks before he launched his war of choice, Tony Blair told the Guardian: “Let the day-to-day judgments come and go: be prepared to be judged by history.” Well, that judgment has just been handed down, and it is utterly damning.

Blair, along with his government and security services, Chilcot concludes, presented the severity of the threat posed by Iraq’s supposed weapons of mass destruction with “a certainty that was not justified”: in other words they sexed up the evidence. Their “planning and preparations for Iraq after Saddam Hussein were wholly inadequate”. They ignored warnings – which proved to be horribly prescient – that “military action would increase the threat from al-Qaida” and “invasion might lead to Iraq’s weapons and capabilities being transferred into the hands of terrorists”.

Blair’s claim that the catastrophe he caused in Iraq could not have been anticipated was demolished with a statement that could serve as the motif for the whole report: “We do not agree that hindsight is required.” All the disasters that came to pass were “explicitly identified before the invasion”.

But the most damning and consequential judgment of all was the one with which Chilcot’s statement began: “We have concluded that the UK chose to join the invasion of Iraq before the peaceful options for disarmament had been exhausted. Military action at that time was not a last resort.”

This is as clear a statement as Chilcot was permitted to make that the war was illegal. The language he used echoes article 33 of the charter of the United Nations, which lays out the conditions required for lawful war. He has, in effect, defined the invasion of Iraq as a crime of aggression, which was described by the Nuremberg tribunal as “the supreme international crime differing only from other war crimes in that it contains within itself the accumulated evil of the whole”.

As Geoffrey Robertson points out, as a result of the long delays in the incorporation of the crime of aggression into the Rome statute (which underpins the international criminal court), there is no legal basis for prosecuting Blair on this charge either in Britain or before the ICC. But there might be other means of achieving the same ends. Six weeks ago, an unprecedented trial concluded in Senegal, where the former ruler of another country – Hissène Habré of Chad – was convicted of crimes against humanity.

An academic survey of 90 countries found that around a third of them have, in one form or another, incorporated the crime of aggression into domestic law. Following the precedent of Habré’s trial, is there a legal reason why Blair should not face a similar process, if, on one of his many lucrative stops around the world, he sets foot in such a nation?

Legal reasons, of course, are not the same as diplomatic reasons, and we can expect the UK and US governments to use a wide range of threats and powers to thwart the principle of equality before the law: after all, international law is what powerful nations do to weak ones. Look at the £600,000 Cameron’s government has spent so far to block a civil case against the former foreign secretary, Jack Straw, and the former head of MI6, Sir Mark Allen, over the kidnapping and deportation to Libya of dissidents from Gaddafi’s regime, who were repeatedly tortured on arrival.

Justice is inseparable from democracy. If a prime minister can avoid indictment for waging aggressive war, the entire body politic is corrupted. In the Chilcot report, there is a reckoning, firm and tough and long overdue. But it’s still not justice.