A clear US commitment to a complete, irreversible withdrawal from Iraq may now be the only way to develop a regional concert of powers that could work with Iraqis to try to stabilise the country and cauterise the conflict.

The continuing US and British occupation is a roadblock to that co-operation. The galvanising impact of a decision to depart unequivocally can be the last best chance at preventing the conflict from boiling over beyond Iraq to the whole region. How we design and implement our departure is our last significant remaining leverage.