Both Jeremy Corbyn and Alex Salmond have already hinted that their response to Chilcot will be a wish to put Tony Blair in the dock – a spectacle long advocated by Desmond Tutu, George Monbiot and others. This hypothetical, however engaging, is a legal impossibility. Instead, we need to concentrate on how the law should be changed to ensure that future leaders who wage wars of aggression can be brought to account.

There is little doubt that the US/UK invasion of the sovereign state of Iraq in 2003 was an unlawful breach of the UN charter, which permits such force only in self-defence, ie when there is an imminent threat of an armed attack, or (less certainly in 2003) a case for humanitarian intervention. But a breach of the charter does not mean that those who lead it are guilty of a war crime – which must, like any other crime, be both clearly defined and justiciable by an available court.

At Nuremberg in 1946, the Nazi leaders were convicted of the crime of aggression as a “crime against peace”, and the waging of an “aggressive” war remained an international crime by the time of the Rome statute, which in 1998 set up the International Criminal Court, opening in 2002. But its jurisdiction to bring aggressors to justice was postponed. Article 5 of the convention says that the court shall not “exercise jurisdiction over the crime of aggression” until member states agree on its modern definition. They did not agree until 2010, and then decided that offenders should not be prosecuted under it until further agreement in 2017.

Might Blair then be prosecuted? No, because it is a fundamental principle that the law is not retroactive: there was not a defined crime in 2003, and it was not within the power of the ICC to do anything about it. George W Bush (the principal offender) and Blair (an accomplice) cannot be brought to any real court of justice for this offence.

As the crime is now defined, they would have a case to answer. It means the planning or preparation or initiation, “by a person in a position effectively to exercise control or to direct political or military action of a state” of an act of aggression, “which, by its character, gravity and scale, constitutes a manifest violation of the charter of the UN”.No doubt Blair could raise defences, and Chilcot might even help (at least in mitigation), if his report finds that Downing Street was misled by incompetent intelligence and the prime minister genuinely believed that Saddam Hussein had been storing weapons of mass-destruction. Chilcot will not help if he finds that inconclusive intelligence was deliberately “sexed up” by the government. But the trial will never happen – Blair broke no law, because there was no law to break. The crime of “aggression” had been put in a state of suspended animation.

Blairites still argue that the invasion was an exercise in humanitarian intervention – it was nothing of the sort

Might Blair be prosecuted next year, then? Might the Nuremberg principle have seeped into the British common law, thus creating a domestic offence for which he could be indicted? This ingenious argument was firmly rejected in 2006 by our supreme court. While accepting that the crime of aggression had existed in international law since 1945, Lord Bingham firmly refused to allow it to become part of UK law without parliamentary sanction. “It is for those representing the people of the country in parliament, not the executive and not the judges to decide [what conduct should be criminal].”

In the debate over Chilcot, Messrs Corbyn and Salmond should insist that the UK ratify the ICC aggression amendment. It will come into force next year, after a 30th ratification (by Palestine) last week. Britain’s refusal to do so has been put down to US pressure, which must be resisted. Moreover, Labour and the Lib Dems should insist that parliament legislate so as to make aggression a crime under our domestic law as well.

Chilcot will provide a hindsight view of 2003. Saddam was a tyrant and a mass-murderer, and Blairites still argue that the invasion was an exercise in humanitarian intervention – it was nothing of the sort. Bush was not a humanitarian, and he specifically ruled this out as a justification, choosing instead to rely on a preposterous legal argument that the US was entitled to “pre-emptive self defence” – ie to invade any country that it thought, subjectively, might one day pose a threat to its national interests.

Blair’s attorney general, Lord Goldsmith, rightly rejected this claim for the US right to “regime change”, but came up instead with a pettifogging point that UN Resolution 1441 might allow “all necessary means” to remove Saddam without any further security council motion. He admitted that he was not confident that any court would agree, and no other state did so. Invasion without security council approval took us backwards towards the lawless world described by Thucydides, in which the strong do what they wish and the weak suffer what they must.

Chilcot will look at the rush to war in March 2003, before weapons inspector Hans Blix could report, and may discover whether the intelligence was being “fixed around the fact” of an intention to go to war in any event. Blair convinced most of his cabinet and parliament that Saddam had WMDs – and it is fair to say that Saddam himself contributed to this pretence, in order to keep his real enemy, Iran, in fear. Blair announced that: “Regime change alone could not be and was not our justification for war. Our primary purpose was to enforce UN resolutions over Iraq and WMDs.”

In that case, of course, war having the most terrible consequences, we should have waited to see whether Iraq had (as it turned out) already destroyed its WMDs in compliance with UN resolutions. Irrespective of Chilcot’s findings, I suspect that the one motive for UK involvement was that conveyed to me by a party grandee, hot foot from Downing Street, at a gathering of then-valued Labour supporters, on the eve of the war: “The bloody yanks are going in anyway, and Tony says we have to be with them to exert a restraining influence.” It is not, of course, a defence to say you only joined a bank robbery to stop the gang leader from using a shotgun.

International criminal law is still in its infancy, and to advance it the UK must ratify the amendment that will make the crime of aggression triable by the ICC. As for the make-believe trial of Bush and Blair, in the absence of any court to which they are accountable we will have to look back on Iraq with an echo of Talleyrand’s cynical aphorism : “It was worse than a crime. It was a mistake.”