The news this week that yet another "green-on-blue" attack has raised the number of US soldiers killed in the 11-year conflict in Afghanistan to more than 2,000 will no doubt fuel the growing perception that Nato's counterinsurgency strategy is doomed to failure.
More than 50 troops with the International Security Assistance Force (Isaf) have been killed so far this year in "green-on-blue" violence perpetrated by rogue members of the Afghan security forces, a phenomenon that has been described as the "signature" attack of the conflict. It is such a dramatic increase over last year's total of 35 that it has forced Isaf to restrict joint operations with the Afghan National Army. About 25 per cent of the attacks, which account for about 15 per cent of Isaf casualties, are believed to have been carried out by insurgent infiltrators who in some cases have used falsified documents to obtain security clearance, while the remaining 75 per cent are blamed on policemen and troops who are not affiliated with the Taliban or other militant groups.
The alarming increase in the number of "insider" attacks has been blamed on a complex web of factors such as perceived insults, anger over accidental Quran desecration, the abuse of Taliban corpses and war crimes such as drone strikes on civilian targets. Analysts believe the perpetrators of most of these attacks are motivated more by personal rage than jihadist ideology.
A disproportionately high percentage of rogue troops are from remote parts of Afghanistan's eastern Nangarhar province, where the central government is weak and the Taliban wield considerable influence over the predominantly Pashtun population. Nangarharis strictly adhere to a rigid code of conduct known as Pashtunwali and are deeply sensitive to anything that seems insulting to their customs, religion and values, and may feel duty-bound to exact vengeance for what most westerners would consider comparatively minor transgressions.
Behaviour that could potentially provoke a deadly reaction includes swearing, public urination and asking to see photos of Afghan recruits' wives.
Another pillar of Pashtun culture associated with green-on-blue attacks is Memastya, the hospitality code, which forbids violating the sanctity of people's homes by conducting searches, a common practice in Isaf counterinsurgency operations that has been blamed for some incidents. The mental and emotional strain of combat, substandard food and living conditions, and a lack of decent health care are also believed to be contributing factors.
Some observers believe that the fragile relationship between Isaf and the Afghan army has been put under far too much stress for there to be any real hope of salvaging the counterinsurgency campaign, but perhaps placing a stronger emphasis on cultural awareness could help to reverse some of the damage, or at least prevent this problem from metastasising further. The recent decision by the Afghan ministry of defence and Nato to jointly publish a cultural guide for native troops so that they will be less inclined to take offence to what many westerners consider normal behaviour is a step in the right direction.
Only a tiny percentage of Afghan troops are likely to go rogue, but if the insidious threat this minority poses means Nato forces are unable to get close enough to give the security forces the support they need, the country may be unable to reverse its seemingly inexorable descent into full-scale civil war when the majority of the coalition's troops pull out in 2014.
Gen John Allen, the top commander of US and Nato forces in Afghanistan, summed it up well when he told the American news programme 60 Minutes: "You know, we're willing to sacrifice a lot for this campaign, but we're not willing to be murdered for it."
