The problem in Iraq, we're now told, was a lack of preparation, or the wrong kind of planning, or mistakes in implementation. If only, say the neocons, we had put our man Ahmad Chalabi in charge from the start, the Iraqis wouldn't have felt so humiliated. If only we hadn't dissolved the army, the pragmatists insist, the insurgency would never have taken off. If only the Brits had been running the show, mutter the old Whitehall hands, all would have been different. The problem, it turns out, was not the invasion and occupation of a sovereign Arab oil state on a tide of official deceit, but the way it was carried out.

Meanwhile, we're being subjected to a renewed barrage of spin about the success of the US surge in turning the country round, quelling the violence and opening the way to a sunlit future. In an echo of his notorious "mission accomplished" speech of May 2003, George Bush yesterday proclaimed the Iraq war a "major strategic victory" in the "war on terror".

All this is self-delusion on a heroic scale. The unprovoked aggression launched by the US and Britain against Iraq five years ago today has already gone down across the world as, to borrow the words of President Roosevelt, "a day which will live in infamy". Iraqis were promised freedom, democracy and prosperity. Instead, as Jon Snow's compelling TV documentary Hidden Iraq underlined this week, they have seen the physical and social destruction of their country, mass killing, tens of thousands thrown into jail without trial, rampant torture, an epidemic of sectarian terror attacks, pauperisation, and the complete breakdown of basic services and supplies.

On the eve of war, Tony Blair told parliament that, while there would be civilian casualties, Saddam Hussein would be "responsible for many more deaths even in one year than we will be in any conflict". Amnesty International estimated annual deaths linked to political repression in Iraq at that time to be in the low hundreds - many more were dying from the impact of western-sponsored sanctions. In the five years since, civilian deaths are estimated at anywhere between 150,000 (the figure accepted by the Iraqi government) and a million-plus, with the Lancet's estimate of 600,000 violent deaths in the first three years alone having held up as the most rigorous. After five years of occupation, Iraq is ranked as the most violent and dangerous place in the world by an Economist Intelligence Unit index. Two million refugees have fled the country as a result, while a further 2 million have been driven from their homes inside Iraq. This has become the greatest humanitarian crisis on the planet.

In the western world, far from the scene of the unfolding catastrophe, such suffering has been somehow normalised as a kind of background noise. But the impact on the aggressor states, both at home and abroad, has only begun to be felt: not only in the predicted terrorist blowback finally acknowledged by Tony Blair last year, but in a profound domestic political alienation, as well as a loss of standing and credibility across the globe. How can anyone take seriously, for example, US or British leaders lecturing China about Tibet, Russia about Chechnya, or Sudan about Darfur, when they have triggered and presided over such an orgy of killing, collective punishment, prisoner abuse and ethnic cleansing?

Given that the invasion of Iraq was regarded as illegal by the majority of the UN security council, its secretary general, and the overwhelming weight of international legal opinion, it must by the same token be seen as a war crime: what the Nuremberg tribunal deemed the "supreme international crime" of aggression. If it weren't for the fact that there is not the remotest prospect of any mechanism to apply international law to powerful states, Bush and Blair would be in the dock at the Hague. As it is, the only Briton to be found guilty of a war crime in Iraq has been corporal Donald Payne, convicted of inhumane treatment of detainees in Basra - while the man who sent him there is preposterously touted as a future president of the European Union.

Those who insist that the immolation of Iraq was the consequence of errors in the execution of an otherwise defensible policy are simply evading their own responsibility and culpability. The likelihood of a bloody quagmire was widely foreseen before the attack. The failure to do so by those who launched the aggression reflects a blindly arrogant refusal to accept that people are bound to resist foreign occupation, however much they detest their own government - particularly in a region that has already been subject to decades of destructive western intervention and exploitation.

Now the same voices can be heard arguing against an end to the occupation on the grounds that withdrawal might trigger even worse violence. Of course no stabilisation of Iraq is going to be bloodless, but such arguments fail to recognise that the occupation itself has fostered sectarian conflict in classic colonial divide-and-rule style - the current US sponsorship of Sunni militias is a case in point. As the US military's own surveys show, Iraqis of all religious and ethnic groups believe the presence of foreign troops is the main cause of violence and 70% want them out now. Tellingly, violence in Basra dropped by 90% after British troops withdrew from the city to their airport base last summer. Naturally, the green zone government is against a US pullout, because it wouldn't survive on its own. But only when the occupation forces make an unequivocal commitment to leave will Iraq's main political and military players be compelled to come to an accommodation.

For the future, so long as the disaster of Iraq is put down to mistakes or lack of planning, the real lessons will not be learned, but repeated - as appears to be happening now in Afghanistan. Gordon Brown has at last promised a full Iraq inquiry when British troops are no longer in the firing line. But any more delay to a proper accounting of what has taken place - including, as the Liberal Democrat leader Nick Clegg said at the weekend, the nature of the US-British relationship - will only further corrode the political system. The disaster of Iraq has at least had the effect of demonstrating the limits of imperial power and restraining further US attacks. The danger is, however, that next time they'll just try and do it differently - without the mistakes.

s.milne@theguardian.com