By his own admission, Private Liam Smith was probably heading for a young offender institution before he joined the Army. It was not his first choice of career, the 18-year-old infantryman confessed. But he could not get a job on civvy street: “I used to get in a lot of fights.” Fighting is almost all that he has done for the past few months. But these have been battles of a very different kind.

In the deserts of Helmand, far from his old life of playground arguments, video games and alcopops, Private Smith has learnt what it means to fight for his life and see his friends die beside him.

“This has been a proper beast. It’s the worst thing I’ve ever done,” he says gravely. “But my dad is proper proud of me now. He never was before.”

In Britain his contemporaries are often derided as binge-drinking, drug-taking, ASBO-earning hoodies. Here, on the battlefields of Afghanistan, the soldiers of the PlayStation generation have been blooded in what one senior soldier described as the most intense ground combat for British troops since the Korean War.

Private Smith knew almost nothing about Afghanistan before 12 Mechanised Brigade arrived in April, and only a little more about war. “I wanted to fight, I wanted to shoot people, but you don’t know what it’s like until you do it.”

He nearly did not make it. He turned 18 in January, squeaking into 2nd Battalion The Mercian Regiment’s deployment by three months as its youngest member. Other soldiers yet to come of age had to stay home until their birthdays passed.

Before they flew out, Pete Lewis, Sergeant Major of A Company, fretted over how his young charges would fare. “I’d go to bed and wake up in the morning wondering how they would cope,” he says. “I didn’t know how they would react in war.”

Private Smith got his first taste of combat only three weeks after arriving in Afghanistan. His company came under attack in Sangin and returned fire, wounding a Taleban fighter. They loaded him on to a stretcher but he was already beyond help. The bullet had gone through his buttocks and ripped his abdomen.

“I had to carry the stretcher,” Private Smith says. “It was horrible. All his guts and intestines were spilling out and slapping against my hand.” He shudders. “I think he died just before we got back. I weren’t that bothered. He was Taleban. But it’s something to see a man die for the first time.”

Two months later they moved to Garmsir, where new foreign fighters – mostly Pakistanis, but also Chechens, Arabs and Uzbeks – are said to undergo their initiation before moving north to fight the British inside the so-called green zone, the jungle-like stretch either side of the Helmand River.

Private Sam Murray, 21, was searching suspected Taleban compounds south of Garmsir in the early hours of September 8 when fighting erupted. He was called to help the neighbouring platoon, which was pinned down under fire. His best friend, Private Johan Botha, had been wounded.

“We were told Botha had been hit but they couldn’t find him,” he says. “It was open ground. They told us Botha was in that direction. There was no cover. That’s when it hit me – I was running into fire.”

Private Murray was at the front of the platoon, a little way ahead of Private Smith, when a bullet whizzed by two metres away and hit their sergeant, Craig Brelsford. “He went down beside me,” Private Murray said. “I tried to hold the FFD [first field dressing] to his neck.”

With the Taleban still shooting and only yards away, he laid down fire as the others rushed forward to help to pull their sergeant off the battlefield. “I had to rip his body armour off so we could carry him, but he was dead before we got him to the Vikings [all-terrain vehicles].”

Officers were forced to call off the search for Botha. “If we’d gone any farther, we would have lost more guys. It’s the hardest thing I’ve had to do in my life, to extract without my mate,” Private Murray said.

A team that went back later to get Private Botha had to chase off Taleban who were trying to drag away his body.

Padre Nick Todd, their chaplain, says: “You lose a friend, it brings things into stark relief. It’s one thing to be afraid, but to have someone actively try to kill you? It’s not what most 18-year-olds are concerned about.”

Back at camp Private Murray could not sleep. Every time he closed his eyes, he saw Sergeant Brelsford drop beside him again.

Private Smith, too, was wrestling with what he had seen that night: one friend shot in the leg, who then started laughing hysterically; another badly crushed by a wall that fell on him when a 500lb (230kg) bomb was dropped; and an old friend from Chesterfield who had been hit in the head with a rocket-propelled grenade.

Even before, he had been grieving for the loss two days earlier of his old school friend, Private Ben Ford, 18. “There are a lot of Chesterfield lads out here. Everyone knows each other.”

Private Ross Stevenson, 18, who was sick and missed the battle, felt little better, having heard it unfold over the radio. “To know my mates were out there and I wasn’t there with them, it was hard,” he says.

Seven days later they were back in the base at Gereshk, preparing with the rest of the battle group for the biggest operation of their tour – and Britain’s largest in Helmand since June. Now under way in the Gereshk Valley, 2,000 troops are trying to clear out Taleban strongholds.

In between loading vehicles and packing their kit, they talked about what Helmand had meant to them. No one was looking forward to going into battle again. “I don’t think anyone is too keen,” Private Stevenson says. “It’s playing on everyone’s minds. I haven’t heard anyone say they want to do it.”

Among the soldiers were a couple of fresh-faced 18-year-olds flown out straight after their birthdays. A 22-year-old private, returning to the front line after being wounded and patched up, tried to warn them of what they would face. “I explained to them what it’s like, that they don’t really want to get involved in it, it’s horrible. They believed us, they understood us, but it is something you need to experience for yourself.”

A little older, he still feels he had not grown up until he came to Helmand. Some of the younger men who have fired towards the enemy have only heard them scream but not seen their hits. The older private knows for sure that he has killed. Last month, pushing towards a Taleban firing position in the green zone, he came face to face with an enemy fighter.

“It was only a couple of seconds but it felt like forever while we looked at each other,” he recalls. “I could tell by the expression on his face that he didn’t know what to do. It was, do I go back to the position or do I go on running? And in that space of time I shot him. You could see when the rounds hit him, you could see by the expression how much pain he was in. That’s the thing that will stick with me for the rest of my life. The look on his face.”

Private Murray is content not to know whether he killed anyone that night. Private Smith would have liked to know that he had, “not to have taken someone’s life, but to have paid them back for my mates”.

When he heard that he would be going out on an operation again, he called his father, who had visited his critically wounded friend in intensive care. “I phoned my dad. He said, ‘You go get revenge for him’. People say they died for their country but they died for their mates. We fight for each other.”

Padre Todd, who has counselled many of the young soldiers, doubts such flashes of bravado. “They aren’t vengeful on the whole,” he says. “There is no bloodlust.”

Instead, they come to share more philosophical thoughts: what it means to lose a friend; whether it is right to kill; or to ask if it is hypocritical that they pray when they are afraid even when they have never been to church.

“It is life-changing, what they are going through,” the chaplain says. “Some have never even been on an aeroplane. There was a lad that came in last week and his first trip overseas was on a TriStar from Brize Norton.”

Some ask whether it is all worth it. Private Murray says that it pleased him to see civilians come back into Sangin after they had cleared the town. Private Stevenson is more sceptical of what his friends’ blood has bought. “I don’t really think we’ve made much difference to the welfare of the country,” he says. “I think this will go on for years and years.”

They are questions that the chaplain believes they should be asking, even if answers are elusive. “If you’re putting your life on the line you have to feel it’s the right thing,” he says. “At the end of the day they’re here because they’ve been told to be. And that probably has to be enough for them.”

At Gereshk, Sergeant Major Lewis busies himself psyching up his boys for their last push. He has nothing but praise for how they have coped.

“I brought out a mixed company – some were boys, some were men. They’ll all be going back as men. If you took the company and split them up, I’d have some right idiots. Together, they’re awesome.”

Brigadier John Lorimer, the top British commander in Afghanistan, says that he barely recognises these teenagers from the photographs he saw of them in training on Salisbury Plain with their white, slightly podgy baby faces. “You see the same soldiers today, they are thin, gaunt. Some of them look like Belsen victims. They’ve lost a hell of a lot of weight.

“They have a real sense of desire in their eyes to do what they’ve been doing. They’re hard. And I think mums and dads, when they get home, they’ll see a completely different [son]. They’ve matured, they’ve aged hugely, and they’ve gained experience that their predecessors in the past few years have never had, never had.”

Padre Todd says: “They probably will have more in common with the older members of the British Legion than they will with the generations in between.”

Their experience in Helmand will doubtless distance them even further from their civilian contemporaries. How will it be to go home to friends whose idea of visceral combat is Street Fighter II?

“When I go home, I don’t act how I used to,” Private Murray says. “What I’ve seen out here, it’s so different. I do find it hard to socialise with other people now. Nothing is the same. They’ll never understand totally what I’ve been through.”

Private Smith plans to visit his mate in hospital and then get back to work. In 18 months he and his battalion will return to Afghanistan for their second tour. He has no plans to change career. “I can’t see myself doing a normal civilian job. Not after this. I think I’ve matured a bit. I used to be a bit of a clown. Not so much any more.”

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