It is hard to think of a more dangerous situation in Iraq right now than to have disenchanted Sunni Arabs believe that the police forces are in the control of Shiite gunmen who can detain and torture Sunnis at will. So the Bush administration made a good start by insisting on an immediate investigation of all Iraqi detention centres after the discovery of what looked like a Shiite militia-run torture chamber in the heart of Baghdad.

Doing that meant brushing aside objections by the Interior Ministry, run by a powerful Shiite leader, and putting a scratch in the thin veneer of Iraqi self-governance. But it is vital to show support for ordinary, law-abiding Sunnis. Still, it’s painful to think of how much easier this would all be if it were not for America’s tarnished reputation when it comes to torture. There was a hollow ring to last week’s talk by American officials of a zero-tolerance policy on the abuse of detainees in Iraqi-run prisons.

It took just a few days for the US to demand a full investigation of Iraqi prisons. Washington recognised that the Iraqi government could not investigate itself alone and assigned the Justice Department and the FBI to help. This same administration, however, has spent the last 18 months obstructing the Senate’s inquiries into Abu Ghraib and other aspects of the prison abuse mess.