One incident, however gruesome, cannot define the way ahead for a military campaign. The Afghan policeman who sprayed soldiers from the British task force in Helmand with rounds from his machinegun may have been a wild card or a Taleban plant, but the death of the five men does not spell the end of Britain’s mission.

Nevertheless, there are some fundamental concerns about the shooting on Tuesday afternoon that go to the heart of the political strategy for Afghanistan to which Britain and the rest of Nato are signed up.

The British exit plan is based not on a set timetable but on the ability of the Afghan National Army (ANA) and Afghan National Police (ANP) to take over responsibility for security in Afghanistan.

Afghanistan, however, is still a long way from becoming security self-sufficient, and the killings at the checkpoint compound at Nad-e Ali have underlined the potential dangers of relying for an exit strategy on imbuing Afghans with the sort of training doctrine normally associated with Catterick and the Royal Military Academy Sandhurst. The cultures are too different.

Since more than 3,000 troops from 16 Air Assault Brigade were deployed to Helmand in 2006, British personnel designated to train and mentor ANA and ANP recruits have often been exasperated at the lack of professionalism and indeed, on a number of occasions, the lack of courage in the face of the enemy.

The weakness in the system — particularly with the ANP — is the vetting of recruits, just as it was in Basra in 2003 and 2004. Thousands of Iraqis were recruited into the corrupt Iraqi police force, with the bare minimum of personal checking by the British military, because of the perceived urgency of training locals to handle security in the city.

As a result, Iraqis with 100 per cent loyalty to extremist militia groups joined the police force and proceeded to use the information they gleaned from the inside to launch roadside bomb attacks on known British patrol routes.

The Taleban will have learnt that lesson and unless the vetting procedure for the ANA and ANP is rigorous, the insurgents will succeed in infiltrating the Afghan security set-up. They may well be behind Tuesday’s shooting.

The timing could not have been worse. With President Obama still unsure of his direction in Afghanistan, the gunman, whether Taleban or not, has exposed the delicate nature of a partnership upon which the whole of Nato’s International Security Assistance Force — and the President himself — is depending.

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