If there were any lingering doubts about whether the former US defence secretary, Donald Rumsfeld, should be indicted before a criminal court, evidence that he asked a veteran of American dirty wars in central America to help set up vicious sectarian militias in Iraq should end them once and for all.

A Guardian investigation reports that Colonel James Steele, a special forces veteran, was nominated by Rumsfeld to help organise paramilitaries to quell a growing Sunni insurgency in Iraq. Steele reported directly to Rumsfeld. The paramilitary groups were drawn from Shia militia and set up detention centres where Iraqis were tortured.

What Rumsfeld and his Pentagon officials knew precisely about these centres is not the only point, or even the main one. These militias would not have been needed, the Sunni insurgency might never have happened, had he worked out even a most elementary basic plan for what to do after the invasion of Iraq. Having ripped responsibility from the US state department for post-invasion planning, his first crime was to fail to take any responsibility himself.

Rumsfeld's notorious "stuff happens" response to looting rampages in Baghdad reflected the most cynical complacency. In evidence to the Chilcot inquiry, Lieutenant General Sir John Kiszely, the UK's senior military representative in Baghdad at the time, quoted Rumsfeld as saying growing attacks in 2004 were the work of a "bunch of no-hopers".

Rumsfeld's covert decisions, now exposed, suggest such complacency was mere rhetoric.

His failure to take on the responsibilities of an occupying power – that was what US and UK forces were, in law as well as practice – was in clear breach of obligations laid down in the Geneva conventions.

His appointment of Paul Bremer as head of the so-called coalition provisional authority – in effect governor of Iraq – and their decision to banish Ba'ath party members and the Iraqi army, compounded the felony, leaving a huge and dangerous vacuum which was filled by assorted criminals, insurgents, and the kind of Shia militia exposed in the Guardian.

Some of those Shia militia attacked and killed British troops in Basra who were themselves in part the victims of a failure to adopt any kind of coherent or responsible plan about what to do after the invasion.

Senior military commanders made it quite clear to this writer soon after the invasion that they believed there was sufficient evidence to indict Rumsfeld and his cohorts. Strong criticism of Rumsfeld and Bremer is a theme running through evidence to the Chilcot inquiry, in autobiographies and in reports by Commons committees.

The next question is how far is the then British government, and Tony Blair in particular, implicated. His senior military advisers, and senior Whitehall officials, vented their anger and frustration, first privately and later more publicly, at Rumsfeld and his Pentagon. Blair, his foreign secretary, Jack Straw and defence secretary, Geoff Hoon, may have vented their frustration too. There is no evidence they actually did so, indeed that they would even have dared, as far as Washington was concerned, to say boo to a goose .

In any case, Rumsfeld was in charge, and was allowed to do anything he pleased, whatever his obligations under international law, and whatever those in charge of America's closest ally, Britain, might have said.