"It's the hardest thing to do," he added. "Send a patrol out - a young man gets killed. I ask, ‘What has that achieved? They went out three kilometres to a village and came back with one man dead.' If you thought about it too much, you'd never actually do this job."

The potential loss of a group of soldiers in a single incident in Helmand, for example the downing of a Chinook helicopter carrying troops, is regarded as having such a cataclysmic effect on public support as to cause the army a strategic reverse. Some operations are called off altogether if weather conditions are judged too poor to allow helicopters to fly casualty evacuation missions.

"They didn't worry about the wind and cloud on D-Day, did they?" a Royal Marine remarked to me in Helmand last year after a battle group operation was aborted minutes before H-Hour due to poor visibility. This caution has further complicated killing the Taliban, an act already beset by paradox. Commanders wishing to preserve the lives of British troops engaged in firefights make frequent use of artillery and air power to destroy the enemy. Indeed, the expenditure of ammunition by British units in Afghanistan is massive compared to past conflicts. But shelling and air strikes raise the likelihood of civilian deaths, inflaming local opinion and at home undermining support for the military's deployment.

Furthermore, the insurgent may be only a "Tier 3" fighter, a local Pashtun farmer lacking militant ideology or commitment. Under the tribal code of Pashtunwali, killing him will ensure that the men of his family are obliged to take up arms against the British. The removal of one fighter will have resulted in the creation of several more.

"I would far prefer a Tier 3 Taliban to run away and come back to till the fields in the summer than have him killed and his family turned against us," Major Chris Bell, Scots Guards, briefed his men before an operation last January, using words that are now a familiar refrain among commanders who want their troops to "de-escalate" and "disengage" from unnecessary firefights with the Taliban.

16 Air Assault Brigade, regarded as one of the army's most prestigious units, finished its latest six-month tour of Helmand at the beginning of October. Their first tour in the province two years ago, "Herrick 4", became infamous not because British units were ever beaten in the fighting but because of the number of Taliban they killed in areas where fighting was of little relevance to the bigger picture. Out-of-step elements of the army crowed over the triumph of British grit and steel in remote outstations such as Naw Zad and Musa Qala, failing to notice the self-defeating nature of the action, which alienated Pashtun tribes throughout Helmand. Support for the British died in the rubble along with the insurgents.