If a soldier shoots his comrade, he’s nearly always under too much pressure

Blue 25 is an unremarkable police checkpoint in central Helmand.

On patrol last week with soldiers from 1 Scots Battlegroup and the Afghan National Army, I wouldn't have noticed it if the patrol commander hadn't pointed it out as the site of the shooting last November of five soldiers from the Grenadier Guards by one of the policemen they were training.

In the aftermath, some questioned the strategy of working alongside the Afghan national security forces. Those of us with experience of operating with the Afghans replied that such incidents were extremely rare and that the vast majority were brave and determined.

The miserably similar killing of three men of 1 Royal Gurkha Rifles this week by an Afghan soldier will not only come as a cruel blow but will also reignite the debate about the strategy of "partnering" and the capability of the Afghan national forces to take over the responsibility for security when the International Security Assistance Force begin to withdraw.

A manhunt is under way for the guilty soldier. The insurgents have, inevitably, claimed him as one of their own but the revelation that he is a Hazara — an ethnic group persecuted by the Taliban and who have made generally more reliable soldiers and policemen than the more ambiguously aligned Pashtuns — suggests that this was the act of a broken individual.

In a way, the motivation is irrelevant. The trust built up between partners has been violated and we are already being told this illustrates the lack of preparedness of the Afghan forces and a major setback.

While I would concur with the former, it does not have to be the latter.

Soldiers turning their weapons on their comrades has a regrettable pedigree. From the shooting of the commanding officer by a man he had ordered flogged as far back as 1815 during the Battle of Quatre Bras to Americans "fragging" officers during the Vietnam war, such incidents invariably indicate are men under tremendous pressure.

Common criticisms of the Afghan army are that they are ill-disciplined and insufficiently motivated. These are problems associated with forces under stress, considerably fatigued and with low morale.

The ANA units in Helmand province have been there for four years. While British brigades rotate through Afghanistan on a roughly six-monthly basis and their American counterparts conduct 12-month tours (which many psychiatric experts assess to be too long), the Afghan soldiers spend their entire military careers on the front line.

Recruits join for a minimum three-year term and are entitled to one month's leave a year. The more experienced Afghan soldiers I was with last week had been in Helmand for more than four years.

During the Army's involvement in Northern Ireland there was a rotation system with units deployed sometimes for as little as six months, sometimes for up to two years, but always knowing at the end they would return to their home bases.

That no such system exists in Afghanistan means that the soldiers posted to the dangerous south are increasingly overworked and suffer disproportionate casualty rates in comparison with those posted to the relatively peaceful north.

The flaws in the current system of deployment neither excuse nor justify the cowardly attack on the Gurkhas. If, however, this serves to highlight the strains under which our Afghan allies are operating, and if Kabul can be persuaded to do something about it, then these tragic deaths may not have been in vain.

Patrick Hennessey served in the Grenadier Guards in Afghanistan in 2007, where he was commended for gallantry. He is author of The Junior Officers' Reading Club