When Captain Leo Docherty of the Scots Guards entered Sangin in May as part of the first British force to seize the Taliban stronghold, he was horrified to discover that they had so little intelligence they did not even know the location for the district chief’s headquarters which they were supposed to secure and use as their base.

“We fought our way into town then were literally asking people where the building was,” he recalled. “Our intelligence was zero. Absolutely f****** zero.”

His Carry on up the Khyber story of Sangin epitomises how what was meant to be a low-risk reconstruction mission has degenerated into the bloodiest combat faced by British troops since the Korean war. Since June, the small dusty town with a dry riverbed through its centre has been the scene of some of the heaviest fighting in Helmand and the graveyard of six British soldiers.

There, as in other small towns in northern Helmand such as Nawzad, Musa Qala and Kajaki, British forces have been under siege. Often running out of food because of the dangers of resupply, they have been forced to drink canal water, and left fighting hand to hand or reliant on bombing from the likes of A10 tankbusters. “Every single location where we have troops is now coming under attack every single day,” admitted a senior MoD official.

Docherty’s experience left him so disillusioned that he quit the army on his return to Britain last month. “I don’t want to be involved in this big clumsy operation that’s going on, which is lethal but pointless,” he said. “It’s completely barking mad.”

His damning first-hand account of life in the field confirms the misgivings of many in what has been the worst week for the British forces since they deployed in Helmand, losing 19 men in six days. It was also by far the most violent week in Afghanistan since the fall of the Taliban in 2001. On Friday, the deadliest suicide bomb in Kabul since the removal of the Taliban left 18 dead, including two US soldiers. This was the second car bomb in the Afghan capital in four days. In a crisis meeting in Warsaw Nato military chiefs pleaded for 2,000 extra troops.

Yet according to Docherty the problem is not just lack of troops and resources but also ill-thought-out strategy and lack of local knowledge. He is well positioned to comment, having worked as aide-de-camp to Colonel Charlie Knaggs, commander of the Helmand taskforce. He then led a team advising the Afghan National Army (ANA) that accompanied British troops to take Sangin. “Col Knaggs said at the beginning that we needed to contain the Taliban and not get sucked in,” he said.

“We had all these study days before deploying, looking at how we dealt with the Malaya insurgency of the 1950s and how we were going to use the same strategy of first creating these secure zones or ink-spots around the main locations of Lashkar Gah and Gereshk and then move out. The whole focus was supposed to be not high-intensity fighting but construction of a nation state. Instead we’ve deviated spectacularly from the plan and scattered in a meaningless way across northern towns of Helmand. To withdraw from these now would be seen as defeat, but the only way to survive is to increase the level of violence.”

Docherty points to Sangin as where it all started to go wrong. He was based at Forward Operating Base (FOB) Robinson, mentoring the ANA, when the orders came to “seize Sangin and re-establish governance”.

Their force was so under-equipped that they had to borrow an old Ford pick-up to transport all the men and had no night- vision goggles. But they nevertheless managed to enter the town on May 25 via the wadi or dry river bed that runs from the east to its centre. Sangin is the centre of the narcotics industry in what is the biggest poppy-growing province in Afghanistan, but there was little fighting as most of the Taliban had melted away. Even though an 11-year-old boy was killed in the capture of the town, accidentally shot through the head, locals were initially friendly. Once they had located their intended base, the British were able to go into the bazaar buying mangoes and fresh nan bread.

“It was a completely wasted opportunity,” said Docherty. “We had a two-week window where we could have done something and shown local people this is why we’re here. But the military is just one side of the triangle and there was no representation by any of the other players (the Foreign Office or Department for International Development). DfID should have been there saying this is the plan, we’re going to sink these wells, pave the bazaar, build this bridge. But they would not come because to them it’s insecure, so they left us unable to offer anything. We did not even get any Information Ops people to explain to locals why we were there. Eventually the Taliban came back and all hell broke loose.”

By late June the British paratroopers who arrived as reinforcements were under siege inside the small compound. “All they were doing was absorbing bullets,” said Docherty. “The only way they could survive was through cannon fire from A10s. FOB Robinson was pounding the town with 30 rounds of heavy explosive from 105mm guns every night. These are hardly surgical tools and I shudder to think of the civilian casualties.”

“All those people whose homes have been destroyed and sons killed or who have had to flee are going to turn against the British,” he said. “It’s a pretty clear equation — if people are losing homes and poppy fields, they will go and fight. I would. We’ve been grotesquely clumsy — we’ve said we’ll be different to the Americans who were bombing villages, then behaved exactly like them. To my mind we’ve lost the hearts and minds before we’ve even begun.”