In a tent in a refugee camp in Dohuk, at the base of ochre-coloured mountains, Amir and Ahmed, 15 and 16 respectively, unroll their mats on the floor to forget the terrible memories. It's a cool autumn evening, and they prop themselves up with cushions and turn on the TV. The two brothers fled from captivity at the hands of the Islamic State in Iraq and Syria (ISIS) six months ago. Now all they want to do is watch some cartoons.

The brothers flip through the channels. ISIS also has its own propaganda station, which viewers can easily receive here, in Kurdish northern Iraq. Ahmed, holding the remote control in his hand, suddenly calls out: "There we are, Amir! That's us!" The brothers recognise themselves on the screen: dressed in black outfits, their faces masked, with other child soldiers during combat training in Mosul.

It is now spring, and that evening spent in front of the television is a few months in the past. Ahmed and Amir, sitting close together, talk about it in the same small tent in the camp. Ahmed speaks in a hoarse voice about their time with ISIS, while Amir stares at the floor. "They gave us drugs, and we believed everything after that."

Ahmed and Amir were held hostage by ISIS for nine months, imprisoned in a military camp in Mosul, its stronghold in Iraq. There the terrorist organisation used beatings and weapons to train them to become child soldiers, or "lion cubs of the caliphate", as ISIS puts it. The "lion cubs" blow themselves up to kill supposed infidels. They witness beheadings, to learn how they are done. They donate blood for injured adult fighters. And they turn in traitors.

It is difficult to determine how many child soldiers ISIS is training. Experts estimate that 1,500 boys are serving the terrorist group in Iraq and Syria. Some are born to the militants. In fact, more than 31,000 women are currently pregnant in ISIS-held territory. Other children arrive with their parents from abroad when the parents join the extremist movement. In many cases, the "lion cubs" are children of local fighters or orphans who join ISIS voluntarily. Others, like Ahmed and Amir, are kidnapped.

The brothers grew up in a village in the Sinjar region. They had a good life, they say, playing soccer, climbing mountains and catching chickens, until ISIS attacked their village in August 2014. The men drove into the village in pickup trucks, threatening the inhabitants until they ran away in fear - but by then it was too late. They pulled the brothers onto a vehicle and drove them to a collection point in Tall Afar, where a decision was reached on how they would be used.

ISIS divided the boys into two groups. The younger, weaker ones were to remain in school to learn the Quran. The older boys were sent directly to military training in Mosul. Ahmed and Amir were taken to a training camp with 200 children. ISIS wanted them to forget that they were Yazidis. They kept their mouths shut, they say, too afraid to say anything.

Ever since ISIS began losing territory in Syria and Iraq due to increased military operations against it, the terror organisation has beefed up its propaganda efforts aimed at children. Researchers with the Quilliam Foundation, a British think-tank that analyses ISIS propaganda, found that last year, significantly more children were being used to promote the terrorist organisation in the media than in the past. The acts of brutality committed by children were also on the rise, with more and more documents surfacing that showed children serving or seeming to serve as executioners for ISIS. It was an attempt to demonstrate resistance against the air strikes in Syria, says Ms Nikita Malik, who headed the study.

By depicting children, says Ms Malik, ISIS wanted to show it was relatively unimpressed by bombs. ISIS' message, she explains, is this: "No matter what you do, we are raising a radicalised generation here."

Within the system, says Ms Malik, the children's task was to spread ISIS ideology in the long term, and to infiltrate society so deeply and lastingly that supporters would continue to exist, even if territory was lost.

THE ESCAPE

In the mornings before sunrise, Ahmed and Amir prayed. Then they learnt the basic skills of a child soldier: How do you disassemble a Kalashnikov? How do you set an improvised explosive device? How do you detonate an explosive vest? ISIS men beat them with sticks and kicked them in the stomach - to make them tougher, as the men put it. In the evenings, the brothers lay on mattresses filled with fleas. Their bodies felt dead, they say. Their thoughts revolved around their mother and father, who they had last seen waving to them as they were taken away on an ISIS truck.

ISIS gave them black Afghan outfits to wear and took them to the front, says Ahmed. They wanted them to see their enemies: the PKK, Yazidis, the Peshmerga. On one occasion, a militant beheaded a Yazidi before their eyes. "We will kill you, too, unless you convert," the men told them. It was during that time that Amir, the gentler of the two brothers, stopped talking.

ISIS also gave them pills, which they didn't want to swallow at first. But then they noticed that the adults who took them seemed more self-confident afterwards. "When we swallowed the pills, everything changed," says Amir. Their fear disappeared, and so did the stabbing sensation in their hearts. The brothers began to believe that Yazidis were inferior.

Every evening, after training, Ahmed slipped away to some bushes behind the dormitory. He had hidden a mobile phone there, on the ground under some branches, which he used to send messages to his mother: "Hello Mama, we're alive", or "I miss you".

When a guard caught him with the banned phone one evening, he says the man dragged him into a room, tore his shirt from his torso and beat him 250 times with the wooden section of a water pipe, breaking Ahmed's sternum.

The bone grew back together crookedly, and today it protrudes from his T-shirt like a boil, a mark that will remind him of the evil of ISIS for the rest of his life. His eyes begin to flicker when he describes the incident.

Eventually they mustered the courage to flee the camp. Together with two children, they ran away at night when the guards weren't paying attention. They travelled for nine days, until they encountered some Peshmerga fighters. They slept under bushes during the day, and they had very little to drink. One of the boys in the group had already tried to escape once before. The ISIS militants shattered one of his feet with the butt of a rifle, and now the other boys had to carry him much of the way.

Former Iraqi dictator Saddam Hussein also abused children as soldiers. Starting in the mid-1990s, military summer camps were built in Iraq for thousands of boys. The regime sought to militarise them to gain better access to society. The unit of child soldiers that took shape after the Iran-Iraq War was called Saddam's Lion Cubs. It has been estimated that several thousand children fought in the Iraq war in 2003.

ISIS later adopted the term, as well as the basic elements of the training programme for Saddam's "lion cubs".

BECOMING CHILDREN AGAIN

Inside the camp, a small crowd gathers at the entrance to the tent where Ahmed and Amir are staying as they tell their story. Their mother, who is sitting with them, wants her sons to become children again. She brings them water when they cry at night. She gently places her hand on their mouths when they quote the Quran. And she hopes that the voices of people being tortured will begin to fade in her sons' heads as time passes.

The recruitment of children takes place in several phases, beginning with harmless socialisation. ISIS hosts events in which children are given sweets and little boys are allowed to hold an ISIS flag. Then they are shown videos filled with violence. Later, in the free schools ISIS uses to promote the movement, they learn Islamic knowledge and practise counting and arithmetic with books that use depictions of tanks. They practise beheading with blond dolls dressed in orange jumpsuits. With a new app developed by ISIS, they learn to sing songs that call upon people to engage in jihad.

In areas controlled by ISIS, there is no alternative to these schools. They are collection points for children that ISIS selects for its military camps. Scouts visit the classes and determine which pupils will become "lion cubs". Once children are caught within the system, it is difficult for them to extricate themselves from it.

It has been three days since Wahad, 11, successfully escaped from an ISIS Quran school in Tall Afar. Now he's sitting on a chair in the office of an aid organisation in Dohuk, a slender boy with blue eyes, red hair and freckles. His Uncle Idriz, an imposing man with a thick moustache, brought him here. He too almost lost his life under ISIS.

Very little support is available for child soldiers who manage to escape. Mr Mirza Dinnayi, an employee of Air Bridge Iraq, gathers their data in the hope that an aid project will be established for them one day. "But at the moment the authorities have enough on their hands with the women raped by IS," says Mr Mirza. Nevertheless, he records Wahad's story.

Mr Mirza has a page of questions he wants Wahad to answer. But Wahad can hardly concentrate. He can't stop thinking about a verse from the Quran. Mr Mirza wants to know how far ISIS managed to pull Wahad onto its side, and the extent to which he was converted.

"Do you speak Arabic?" he asks in a friendly, eager tone.

"No," says Wahad.

"What do you speak?"

"Kurdish," says Wahad.

"And how do you read the Quran, since it's in Arabic?"

Wahad is silent.

"Are you a Muslim or a Yazidi?"

Wahad looks at the floor, his cheeks flushed with shame. "Yazidi," he says, in barely a whisper. He can hardly bear the idea of having betrayed the Yazidis, and of being on the wrong side.

Wahad also stood in front of the school in Tall Afar when ISIS was sorting its hostages. He was one of the weak ones who were sent to the Quran school. ISIS locked him and 34 other boys into a bare room that became their prison for the next 20 months.

Every morning before sunrise, a female ISIS teacher would go to the room to wake them up to pray. After that, they studied the Quran for seven hours. To encourage them to apply themselves, they were promised, as if they were in a fairy tale: "If you're good, you will be allowed to see your mothers."

Wahad did not become a soldier, and he didn't fight, and yet he is so traumatised that he hardly speaks any more. Psychologists at the Mental Health Centre in Dohuk work with children like him, "unless the parents themselves are too traumatised to bring them", psychiatrist Thikra Ahmed Muhammed says in a colourful playroom.

In the past, children who had experienced the trauma of car accidents or bed-wetters were the ones receiving therapy. Now many of the patients have depression, and some of them, even children younger than 10, have attempted suicide.

A four-year-old child, the psychiatrist explains, tries out the things he has seen on his six-year-old sister: putting out cigarettes on her arm or tying a rope around her neck. A five-year-old boy wakes his mother in the dark and says: You have to pray.

Dr Thikra paints pictures with the children. They often draw themselves, with large eyes and small mouths.

In the evening, Wahad is standing in front of one of the apartment buildings in Dohuk where he and his uncle are staying. Wahad watches the neighbourhood children playing with marbles, but he keeps gazing off into the distance. His Uncle Idriz, sitting next to him on a bench, describes how ISIS killed a child that tried to escape and threw the body into the midst of the other children.

Wahad says almost nothing, but when he speaks, his voice sounds like the chirping of a small bird.

His Uncle Idriz is also unable to forget the images from the past. When ISIS attacked his village, he and other Yazidis were forced to lie on the ground in a row. The ISIS militants walked down the line and shot and killed 380 men, but they missed Mr Idriz. He played dead and survived after nomads found him. They fed him tomatoes and raw liver.

It is difficult for Mr Idriz to accept that Wahad has become a silent child, and that Wahad probably will not be able to cope with the horrors as well as him. But Mr Idriz does his best. He goes over to the children playing marbles, puts his hand on Wahad's shoulder and firmly massages his neck.

"I'll take you to the park tomorrow," he says. "You'll see normal life there. I will make you strong, Wahad."

Mr Idriz gives him a little push. Wahad, looking ashamed, picks up a few marbles, looks up at his uncle, leans his head against his stomach and, in his soft voice, says: "Yes, as strong as a warrior."

DER SPIEGEL