There was no single reason why Britain and the US went to war in Iraq. The motives that inspired George W Bush and Tony Blair have been variously dissected, analysed and psychoanalysed. It is too early for history to have formed a settled view on the war, but the case that it was a monumental error gets ever more compelling.

Most of the official justifications for war, on grounds of security from terror and weapons of mass destruction, have been discredited. The only element of moral authority left in the decision might be that Saddam Hussein ran a murderous regime, characterised by torture and extra-judicial killing. It could indeed have been the duty of western powers to intervene against such atrocity. But the western occupiers quickly became complicit in atrocities of their own, as new leaked military documents reveal.

The files, passed to WikiLeaks and reported in today's Observer, reveal how allied forces turned a blind eye to torture and murder of prisoners held by the Iraqi army. Reports of appalling treatment of detainees were verified by the US army and deemed unworthy of further investigation. Responsibility for disciplinary action was passed to the Iraqi units that had perpetrated the abuse. In a handful of cases, allied soldiers are directly implicated in abuse.

The leaked files expose a cavalier attitude towards international law with regard to the treatment of enemy soldiers and disgraceful tolerance of civilian casualties.

The thrust of these allegations is not new. But each extra piece of evidence builds a portrait of a military occupation deeply implicated in practices that were illegal under international law and unconscionable in the eyes of any reasonable observer.

The terrible truth about British and American involvement in Iraq seems increasingly to be that it was not just a strategic failure, it was, for the occupying powers, a moral catastrophe.