“They are fighting in sandals with two magazines of ammunition. We’ve got 100 pounds on our backs trying to chase after them.” Pouring sweat, unwashed for three days, ruddy faced and peeling viciously in the midday sun Sergeant Matthew Parry and Lance Sergeant Colin Skitt, both 31, tried to explain the experience of their unit, the Welsh Guards, over the past two months fighting in the Nad Ali district of Helmand.

The temperature was 40C and the shade in their tiny checkpost was limited to one tree, under which 20 men were packed like sardines, sucking down water, as others stood guard.

“We’ve lost count of the firefights. The small arms fire is like a game of tennis really,” said Sergeant Parry. “But as soon as we get the big stuff in,” he nodded skyward, “they tend to bug out.”

Lance Sergeant Skitt said that the Taleban used pre-prepared weapons caches. “They use the ammo and sprint to the next one. You think you are being engaged by several different positions but it’s just one bloke. Then they drop their weapon and turn into a civilian.”

Its a tough, frustrating, stoic business. It also demands relentless self-discipline and unenviable judgement calls. In another barren checkpoint sat Major Giles Harris, 36, the commander of the Prince of Wales Company of the Welsh Guards, a radio crackling at his side.

“When we meet the bad guys, we win,” he said. “But it is a continual challenge to balance the effect this has on the civilians in the middle.

“It is the discipline required not to take the gloves off. You are asking my guardsman not to empty the magazine of his weapon into the compound wall from which he is being shot at . . . When we call in Apache helicopters they are superb, a great morale boost for the boys. But you think ‘did I do the right thing?’. It only takes one stray cannon round (from the helicopters) and you take two steps back.”

In a tactical directive to the troops under his command the new commander of Nato forces in Afghanistan, General Stanley McChrystal, wrote on Monday: “We must avoid the trap of winning tactical victories — but suffering strategic defeats — by causing civilian casualties or excessive damage and thus alienating the people . . . The Taleban cannot militarily defeat us, but we can defeat ourselves.”

This is easier said than done when soldiers are facing attacks every day from a largely unseen enemy that fires on them from civilian buildings and can within seconds turn into a bewildered and terrified-looking local civilian.

There is often nothing to warn of the Taleban’s home-made roadside bombs. The explosive is made from fertiliser, the triggers usually simple pressure plates rigged from saw blades and wooden blocks wrapped in a car innertube. Such devices are responsible for most of the nine deaths that British soldiers have suffered in nine days.

Given the choice every soldier will take a firefight against rocket-propelled grenades and Kalashnikov fire over a roadside bomb. The bombs are the “worst-case scenario”, says Lance Sergeant Waisale Soko, 28, a Fijian with a soft Welsh accent, compared to the “bread and butter” of a firefight.

The narrow lanes of Nad Ali, an area of dense agriculture and numerous drainage ditches, are perfectly suited to ambush and roadside bombing and there is little chance to vary routes from the few existing roads.

Armoured vehicles could turn into the fields in some places but are forbidden to do so, except in extreme circumstances, because it is believed that local support for Nato forces would be damaged if they destroyed farmers’ fields.

The bombers’ task is therefore made easy and, according to British bomb disposal experts, they are already more prolific and inventive than the IRA managed to become.

In such circumstances patience and discipline are the soldiers’ best defence. Interminable hours are spent sweeping roads and crawling along behind the mine detection crews as the summer heat in Helmand builds higher and higher.

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