I have encountered no sense of vindication, no "I told you so", among veterans of the anti-war protest of 15 February 2003 in response to the events in Iraq. Despair, yes, but above all else, bitterness – that we were unable to stop one of the greatest calamities of modern times, that warnings which were dismissed as hyperbole now look like understatements, that countless lives (literally – no one counts them) have been lost, and will continue to be so for many years to come.

In July 2002, the Guardian warned that Britain was "sleepwalking to war". Blair's commitment to invade come what may – which the Chilcot inquiry (when it is finally published) will either confirm or whitewash – is now established. By September 2002, the inevitability had sunk in. In the first demonstration, hundreds of thousands of people took to the streets in London on 27 September – me and my grandfather among them – full of determination and foreboding. Three weeks earlier, Amr Moussa, then-secretary general of the Arab League, warned that the Iraq war would "open the gates of hell".

I remember the premature triumphalism and hubris of the cheerleaders in the run-up. In my first year at university, one of Britain's most senior army officers came to talk to students as the guest of Lord Butler, who would later head one of the inquiries into the war. When Iraq was invaded by western forces, he told us solemnly, 99% of the Iraqi population would be on the streets, throwing flowers at advancing troops. The other 1% would still be cowering at home, too scared to celebrate, but would be quickly reassured. Men such as this helped direct the entire war effort. And then there those who were not listened to, such as former UN chief weapons inspector Scott Ritter, who warned in 2002 that "since 1998 Iraq has been fundamentally disarmed"; or Robin Cook, who told a hushed House of Commons as he resigned that "Iraq probably has no weapons of mass destruction in the commonly understood sense of the term."

The catastrophic results of the Iraq invasion are often portrayed as having been impossible to predict, and only inevitable with the benefit of hindsight. If only to prevent future calamities from happening, this is a myth that needs to be dispelled. The very fact that the demonstration on that chilly February day in 2003 was the biggest Britain had ever seen, is testament to the fact that disaster seemed inevitable to so many people.

The commentators who cheered on the conflict, far from being driven from public life are still feted: still writing columns, still dispensing advice in TV studios, still hosting thinktank breakfasts. "If nothing is eventually found, I – as a supporter of the war – will never believe another thing that I am told by our government, or that of the US ever again," declared David Aaronovitch in this newspaper.

A few months after the invasion, he wrote: "There have been very few suicide attacks." In the seven years that followed, 12,284 civilians would perish in 1,003 suicide bombings. He went on: "If Iraq becomes anything like a democratic and pluralistic state, then just about everything that the opponents of intervention predicted will have turned out to be wrong. If it descends into long-term chaos and civil war, then just about everything they said will turn out to have been right."

If Aaronovitch was to stay true to his word, he would now be expressing the greatest mea culpa of the century; instead, he has written a column in the Times today [paywall] which makes it clear he has no intention of expressing any regret.

In a way, opponents of the war were wrong. We were wrong because however disastrous we thought the consequences of the Iraq war, the reality has been worse. The US massacres in Fallujah in the immediate aftermath of the war, which helped radicalise the Sunni population, culminating in an assault on the city with white phosphorus. The beheadings, the kidnappings and hostage videos, the car bombs, the IEDs, the Sunni and Shia insurgencies, the torture declared by the UN in 2006 to be worse than that under Saddam Hussein, the bodies with their hands and feet bound and dumped in rivers, the escalating sectarian slaughter, the millions of displaced civilians, and the hundreds of thousands who died: it has been one never-ending blur of horror since 2003.

The invasion was justified as an indispensable part of the struggle against al-Qaida. Well, to be fair, large swaths of Iraq have not been handed over to al-Qaida: they are now run by Isis, a group purged from al-Qaida for being too extreme. Iraq and Syria are trapped in a bloody feedback loop: the growth of Isis in Iraq helped corrupt the Syrian rebellion, and now the Syrian insurgency has fuelled the breakdown of Iraq, too. Those who believe that the west should have armed Syria's rebels should consider the fact that Isis reportedly raided an arms depot in Syria which was stocked with CIA help. Support from western-backed dictatorships in Saudi Arabia and Qatar has fuelled the Syrian extremists now spilling over into Iraq.

Such is the brutal sectarianism of Iraq's Shia prime minister Nouri al-Maliki, that some Mosul residents are reported to be fleeing because they fear an army counterattack; other Mosul residents are even welcoming Isis as a liberation.

What hope, then, for the future? It is difficult to see how the continuing collapse of Iraq can be avoided: the more informed the expert, the more despairing they seem to be. There will be those who champion more western intervention. But whatever happens, this calamity must never be allowed to happen again.