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.

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.

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.