Britain’s Armed Forces are confronted by a challenge “as great as any that have gone before us in the last century,” a conflict across unpredictable battlegrounds that General Sir Richard Dannatt, the Chief of the General Staff, expects to encompass a generation. They are being asked to conduct extended combat operations against “a strident Islamist shadow” with a defence budget that is lower than at any time since the pacifist early 1930s. In Iraq and Afghanistan, it has been tragically clear that British troops do not have all the equipment, notably but not only helicopters, that they need. Lives have been lost unnecessarily, initially from a lack of armoured vehicles.

The military have also been set insufficiently clear objectives. The Government’s decade-old military strategy assumed that the typical mission would be speedy, highly mobile and of short duration. Commanders in the field accept the need to improvise. They should not be expected to “muddle through”, as they have been forced by constraints on manpower and equipment to do in southern Iraq. The men and women sent into Helmand province in Afghanistan should not have been told, as they were by John Reid then the Defence Secretary, that they were on a peace-building mission and would in all probability return home without firing a shot in anger.

Political parsimony has generated military uncertainty. In General Dannatt’s speech to senior officers in June that has only now become public, he underlined how vital was “success” in Iraq and Afghanistan, declaring that failure in either campaign would leave tomorrow “a very uncertain place”. Yet he did not define “success” or set out a long-term strategy for either theatre. He did not even clarify the short-term objectives: what, precisely is meant by “significant achievement” in Afghanistan? Tellingly, his concern was “the need to keep an army in being” along the line, “not just the memory of one that expended itself” in these two theatres.

Armies that are perceived to be in damage-limitation, not winning, mode cannot impose their will. Their enemies harry them to call it a day; it is only a question of when. This has been the British predicament in Basra. The signal that the British were not going to stay the course went out soon after the invasion, when after a few months force levels were halved and then halved again. Under pressure to hand over and get out, the British recruited untrustworthy Iraqis en masse to the police and administration. Basrawis who had trusted the British to keep order ceased to do so and hell broke loose. The city that the battalion holed up in Basra Palace is about to hand over to the Iraqi Army is an all but ungovernable morass of rival militias, smugglers and organised crime.

The temptation will be to say that the remaining 5,000 at Basra airport are no more than a symbolic presence, and pull them out too. But southern Iraq cannot be left to sink or swim. It accounts for 70 per cent of Iraq’s oil reserves, 90 per cent of government revenue and through it run the coalition supply routes from Kuwait. The US will not thank Britain if its forces have to divert south from “surge” operations to fill the gap. What the British quaintly choose to call “reposturing” would be called running for the exit by extremists everywhere, including Afghanistan. It would be a “symbolic absence” of great potency. It would be failure. At President Bush’s side last month, Gordon Brown spoke of “duties to discharge and responsibilities to keep” in Iraq. He must now pronounce those same words at home.