Historians will find it hard to resist labelling September 3, 2007, as the day Britain and the US finally parted company on Iraq. Two high-security dramas that concluded on the same hot morning – one in which President Bush flew into the al-Asad airbase for a “war council” with his top generals, the other the British withdrawal from Basra Palace to the airport – appeared to symbolise two rapidly diverging views on what can still be achieved by coalition forces.

On paper, the evacuation of the palace by the 4th Battalion The Rifles, under cover of darkness and a media blackout, was part of a longstanding plan agreed with US forces to hand over the city to the Iraqi Army. In reality, control of territory and timing in Basra has long since rested primarily with rival Shia militias, and no one can predict with confidence how they will respond to the withdrawal. Yet this much is clear: short of mounting a “surge” of its own, the British Army’s decision was the only realistic one in the circumstances. And while the pullback was hardly triumphant, it was orderly, purposeful and the fulfilment of a promise made four years ago.

Mr Bush’s purpose in touching down in the Iraqi desert was to highlight progress made against al-Qaeda cells in parts of Anbar province, and to ensure that he can evaluate General David Petraeus’s long-awaited report to Congress later this month on the basis of first-hand experience, however brief. Thanks to the continued presence of 5,500 British troops at Basra airport, he will also be able to claim that the core of the coalition that removed Saddam Hussein remains intact. Critics will argue that this face-saving role for Washington is the only real purpose left for British Forces in Iraq. This is not true. They will continue to patrol beyond their base, and in doing so will be well placed to gather intelligence on three vital issues: the relative strengths of the militia groups vying for control of the city, their level of penetration of Iraqi police and army units, and the extent of Iranian influence on local government and clerics.

Such a “to do” list would have dismayed British commanders had it been foretold at the time of the 2003 invasion, and it is dismaying now. There can be little doubt that their professionalism and the noble intentions of the coalition as a whole have been undermined grievously by poor planning and even, as General Tim Cross has alleged, the wholesale rejection of planning. But Gordon Brown’s commitment to “our responsibilities to the Iraqi people” and “our international obligations” must be more than a form of words; it is still likely to mean a sustained if not indefinite military presence.

A phased withdrawal is under way, but its timing must be determined by Iraqi needs and Mr Brown’s assessment, based on his commanders’ advice, of the situation in southern Iraq. It must also be as meticulously planned as the operation that moved 500 soldiers from Basra Palace to the airport without a casualty. There will be no shortage of second-guessing from critics of Project Iraq. Yet they should concede this: it was always the coalition’s goal to hand Iraq’s government and then responsibility for its security back to the Iraqis. For better or worse, that is what is happening. Optimists hoped a thriving new democracy would form part of this bequest. Such hopes have waned, and Saddam Hussein’s removal is now regarded as a consolation, not a justification, for the chaos that followed. History may yet conclude it was more than that.