General David Petraeus, the US commander in Iraq who led the "surge" of 30,000 troop reinforcements over the past 20 months, leaves behind a different country as he departs to take charge of US Central Command. But it is important to understand why, how vulnerable the achievement is, and the enormous task remaining if there is to be any chance of putting a broken Iraq back together again.

Gen Petraeus, to his credit, has recognised that the advances of the surge are both fragile and reversible. There has been a marked reduction in violence, albeit from almost apocalpytic levels, which has transformed Baghdad in particular. But the surge was meant to create the space for political reconciliation, a new national compact and the rebuilding of Iraq. That, quite simply, has not happened.