Hamza is a 33-year-old from Fallujah, a city ruled by Islamic State 40 miles west of Baghdad, who became an Isis fighter last year after being attracted by its appeal to his religious feelings. Two months ago, however, he defected, after being asked to help execute people he knew – and being appalled by invitations to join in what amounted to rape of captured Yazidi women.

In an interview with The Independent, given in the safety of another country, he gives a vivid account of why he joined Isis, what it was like to be a member of the jihadist group, and why he left. He reveals extraordinary details about how the army of Isis operates, the elaborate training that its fighters receive in Iraq and Syria and the way in which taking part in executions is an initiation rite, proof of the commitment and loyalty of fighters.

An intelligent, idealistic, well-educated, and religious man, Hamza defected from Isis after six months as a trainee and a fighter because he was deeply upset by the executions, some of them of people he knew in Fallujah. He became conscious that if he stayed in Isis he would soon have to carry out an execution himself. “I don’t like Shia but when it comes to killing them I was shocked,” he says.

He refused to execute some Sunni accused of working with Iraq’s mostly Shia government “or what they [Isis] call ‘the pagan government’,” he said. Surprisingly, he was not punished for this, but was told by his commander that he would be asked to carry out an execution later and, in the meantime, foreign jihadis would do the job.

Hamza gives a fascinating insight into the lives led by Isis fighters. “I was paid 400,000 Iraqi dinars (£231) a month in addition to many privileges, including food, fuel, and more recently, access to the internet,” he says.

Timeline: The emergence of Isis Show all 40 1 /40 Timeline: The emergence of Isis Timeline: The emergence of Isis 2000 Abu Musab al-Zarqawi (pictured here) forms an al-Qaeda splinter group in Iraq, al-Qa’eda in Iraq. Its brutality from the beginning alienates Iraqis and many al-Qaeda leaders. Timeline: The emergence of Isis 2006 Al-Zarqawi is killed in a U.S. strike. Al-Zarqawi’s successor, Abu Ayyub al-Masri, announces the creation of the Islamic State in Iraq (ISI). Reuters Timeline: The emergence of Isis 2009 Still al-Qaeda-linked ISI claims responsibility for suicide bombings that killed 155 in Baghdad, as well as attacks in August and October killing 240, as President Obama announces troop withdrawal from Iraq in March. Getty Images Timeline: The emergence of Isis 2010 Abu Bakr al-Baghdadi becomes head of ISI, at lowest ebb of Islamist militancy in Iraq, which sees last U.S. combat brigade depart. Timeline: The emergence of Isis 2012 In Syria, protests (pictured here starting in Daree) have morphed into what president Assad labelled a “real war” with emergence of a coalition of forces opposed to Assad’s regime. Syria group Jabhat al-Nusra are among rebel groups who refuse to join, denouncing it as a “conspiracy”. Bombings targeting Shia areas, killing more than 500 people, spark fears of new sectarian conflict. Sunni Muslims stage protests across country against what they see as increasingly marginalisation by Shia-led government. AP Timeline: The emergence of Isis 2013 Al-Baghdadi renames ISI as the Islamic State in Iraq and Syria, or Isis, as the group absorbs Syrian al-Nusra, gaining a foothold in Syria. In response, al-Qaeda chief Ayman al-Zawahiri (Bin Laden’s successor) concerned about Isis’ expansion orders that Isis be dissolved and ISI operations should be confined to Iraq. This order is rejected by al-Baghdadi. AFP Timeline: The emergence of Isis 2014 - January Isis fighters capture the Iraqi cities of Fallujah and Ramadi, giving them base to launch slew of attacks further south. AP Timeline: The emergence of Isis 2014 - June Isis declares itself the Caliphate, calling itself Islamic State (IS). The group captures Mosul, Iraq’s second largest city; Tal Afar, just 93 miles from Syrian border; and the central Iraqi city of Tikrit. These advances sent shockwaves around the world. Timeline: The emergence of Isis 2014 - June Around the same time Isis releases a video calling for western Muslims to join the Caliphate and fight, prompting new evaluations of extremists groups social media understanding. Timeline: The emergence of Isis 2014 - June Isis take Baiji oil fields in Iraq - giving them access to huge amounts of possible revenue. EPA Timeline: The emergence of Isis 2014 - August James Foley is executed by the group as concerns grow for second American prisoner, fellow reporter Steven Sotloff. AP Timeline: The emergence of Isis 2014 - August Obama authorises U.S. airstrikes in Iraq, helping to stall Isis’ along with action by Kurdish forces following the deaths of hundreds of Yazidi people on Mount Sinjar. Timeline: The emergence of Isis 2014 - September Isis release video showing Steven Sotloff’s murder prompting Western speculation his executioner is same man who killed Mr Foley. EPA Timeline: The emergence of Isis 2014 - September Obama tells us that America “will hunt down terrorists who threaten our country” EPA Timeline: The emergence of Isis 2014 - September Isis release a video appearing to show David Haines, who was captured by militants in Syria in 2013, wearing an orange jumpsuit and kneeling in the desert while he reads a pre-prepared script. It later shows what appears to be the aid worker's body. Rex Timeline: The emergence of Isis 2014 - September Peshmerga fighters scrabble to hold positions in the Diyala province (a gateway to Baghdad) as Isis fighters continue to advance on Iraqi capital. AFP Timeline: The emergence of Isis 2014 - October Aid worker Alan Henning is killed. Self-imposed media blackout refuses to show images of him in final moments, instead focuses upon humanitarian care. AP Timeline: The emergence of Isis 2014 - October Isis raise their flag in Kobani, which had been strongly defended by Kurdish troops. The victory goes against hopeful western analysis Isis had overextended itself, while alienating much of the Muslim population through the murder of Henning. Victory causes fresh waves of Kurdish refugees arriving in Turkey. Timeline: The emergence of Isis 2014 - November American hostage, who embarced values of Islam, Peter Kassig and 14 Syrian soldiers are shown meeting the same fate as other captives. But intelligence agencies will be poring over the apparently significant discrepancies between this and previous films. Seramedig.org.uk Timeline: The emergence of Isis 2015 - February Isis has released a video revealing the murder by burning to death of a Jordanian pilot held by the group since the end of December 2014. Reuters Timeline: The emergence of Isis 2015 - February Isis militants have released videos which appear to show the beheading of Japanese hostages Haruna Yukawa and Kenji Goto. Timeline: The emergence of Isis 2015 - February American aid worker, Kayla Mueller was the last American hostage known to be held by Isis. She died, according to her captors, in an airstrike by the Jordanian air force on the city of Raqqa in Syria, though US authorities disputed this. AP Timeline: The emergence of Isis 2015 - February Isis militants have posted a gruesome video online in which they force 21 Egyptian Coptic Christian hostages to kneel on a beach in Libya before beheading them. Egypt vowed to avenge the beheading and launched air strikes on Isis positions. AP Timeline: The emergence of Isis 2015 - February The British Isis militant suspected of appearing in videos showing the beheading of Western hostages has been named in reports as Mohammed Emwazi from London. Rex Features Timeline: The emergence of Isis 2015 - March Isis triple suicide attack has killed more than 100 worshippers and hundreds of others were injured after the group members targeted two mosques in the Yemeni capital of Sanaa. AP Timeline: The emergence of Isis 2015 - April Iraqi forces have claimed victory over Isis in battle for Tikrit and raised the flag in the city. EPA/STR Timeline: The emergence of Isis 2015 - April Isis has claimed responsibility for a suicide bomb attack in Afghanistan that killed at least 35 people queuing to collect their wages and injured 100 more. EPA Timeline: The emergence of Isis 2015 - April Isis’ media arm released a 29-minute video purporting to show militants executing Ethiopian Christians captives. The footage bore the extremist group’s al-Furqan media logo and showed the destruction of churches and desecration of religious symbols. A masked fighter made a statement threatening Christians who did not convert to Islam or pay a special tax. Timeline: The emergence of Isis 2015 - May Abu Bakr al-Baghdadi, the leader of Isis has been "incapacitated" by a spinal injuries sustained in a US air strike in Iraq. He is being treated in a hideout by two doctors from Isis’ stronghold of Mosul who are said to be "strong ideological supporters of the group". Timeline: The emergence of Isis 2015 - May Isis has also claimed responsibility for killing 300 of Yazidi captives, including women, children and elderly people in Iraq AP Timeline: The emergence of Isis 2015 - May Isis attack on Prophet Mohamed cartoon contest in Texas was its first action on US soil. Two gunmen were shot and killed after launching the attack at the exhibition. Elton Simpson and Nadir Soofi have been named as the attackers at the Curtis Culwell Centre arena in Garland. Timeline: The emergence of Isis 2015 - May Isis’s deputy leader, Abu Alaa Afri, a former physics teacher who was thought to have taken charge of the deadly terrorist group, has been killed in a US-led coalition airstrike. Timeline: The emergence of Isis 2015 - May US special forces have killed a senior Isis leader named as Abu Sayyaf in an operation aiming to capture him and his wife in Syria. Getty Images Timeline: The emergence of Isis 2015 - May Iran-backed militias are sent to Ramadi by the Iraqi government to fight Isis militants who completed their capture of the city. Government soldiers and civilians were reportedly massacred by extremists as they took control and the army fled. Charred bodies were left littering the city streets as troops clung on to trucks speeding away from the city. Ramadi is the latest government stronghold to fall to the so-called Islamic State, despite air strikes by a US-led international coalition aiming to stop its advance in Iraq and Syria. AFP Timeline: The emergence of Isis 2015 - May Isis rounded up civilians trapped in Palmyra and forced them to watch 20 people being executed in the historic city’s ancient amphitheatre. The Unesco World Heritage site was overrun by militants, threatening the future of 2,000 year-old monuments and ruins. Thousands of Palmyra’s residents fled but many are still living within the city walls, while the UN human rights office in Geneva said it had received reports of Syrian government forces preventing people from leaving until they retreated from the city. Getty Timeline: The emergence of Isis 2015 - May A group of Isis-affiliated fighters have captured a key airport in central Libya. The militants took control of the al-Qardabiya airbase in Sirte after a local militia tasked with defending the facility withdrew from their positions. Affiliates of Isis, already control large parts of Sirte, the birthplace of former Libyan leader Muammar Gaddafi and a former stronghold of his supporters. Timeline: The emergence of Isis 2015 - June The US Air Force has destroyed an Isis stronghold after an extremist let slip their location on social media. According the Air Force Times, General Herbert "Hawk" Carlisle, commander of Air Combat Command, said that Airmen at Hulburt Field, Florida, used images shared by jihadists to track the location of their headquarters before destroying it in an airstrike. Reuters Timeline: The emergence of Isis 2015 - June Kurdish forces captured a key military base in a significant victory in Raqqa as well as town of Tell Abyad. YPG fighters, backed by US-led airstrikes and other rebels, consolidated their gains, when they seized the key town on the Syria-Turkey border. They are now just 30 miles to the north of Raqqa and have cut off a major supply route deep inside Isis-held territory. Ahmet Silk/Getty Timeline: The emergence of Isis 2015 - June Isis has released gruesome footage claiming to show the murder of more than a dozen men by drowning, decapitation and using a rocket-propelled grenade as it seeks to boost morale among its fanatical supporters. Timeline: The emergence of Isis 2015 - June Isis has begun carrying out its threat to destroy structures in the ancient Syrian city of Palmyra, blowing up at least two monuments at the Unesco-protected site as Syrian government troops made advances on the Islamist’s positions. AFP

“On the other hand, there were some Tunisian Muslim girls who came from Syria. Those Muslim girls were sleeping with some commanders under a marriage contract for a week only and then they were divorced and married to another one. I asked one of them how she had come to be in Syria and she answered that she had travelled first to Turkey and then across the Turkish-Syrian border.” The three British schoolgirls who likewise crossed into Syria may well find that they are similarly treated by Isis.

Hamza does not want his real name or location disclosed, though he believes that for the moment he is safe. He asked for certain details about his escape in January be concealed, but otherwise is open about how he came to join Isis forces and what he did. In many cases what he says can be confirmed by other witnesses from Fallujah interviewed by The Independent, though none of these were fighters.

“It’s a complicated story,” he says, when asked how he came to join Isis. Last year, the group captured Fallujah, where Hamza and his family were living. “They were kind to people in general and did not force them to join their military service,” he recalls. “They had many ways of gaining people’s goodwill and support: for example, they would go house to house, asking those living there if they needed anything and offering services such as education, saying ‘We will enlighten your children, so don’t send them to the government’s schools.’

“In addition, they were giving small lectures and sermons after prayers. Most of the lecture topics were about how to reform and improve society, using the Koran and Hadith [traditional Islamic teachings] to support their arguments.

“This was like some kind of brainwashing but it happened slowly over six months. I was attending many of those lectures and, after a time, I was preparing in advance the Koranic verses and Hadith texts relevant to the topics. There were weekly competitions between groups of youths. I won two competitions on these religious topics and each time I received 300,000 Iraqi Dinars.”

Last July, his family left Fallujah for Baghdad, but he remained behind. “After winning two prizes, I felt I liked their system,” he says. “When my family left, my father asked me not to stay and told me not to be too influenced by the prizes I had won. He said the situation would get worse. He was not very opposed to Isis, but he is so old and cannot cope with the hard life in Fallujah after conditions deteriorated – in terms of work, electricity, water, food, and the militarisation of life.”

Hamza told his family that he would follow them to Baghdad within a few days, but had decided at this moment, July 2014, to join Isis. His motive was primarily religious and idealistic. He says that he “decided to join them willingly because I was convinced that the Islamic State is the ideal state to serve, and to work for, Allah and the after life, which is the surest part of life”.

He was accepted immediately by Isis, his preacher recommending him to a military commander, though he was not at first sent to a military unit. The details Hamza gives of his induction and training by Isis are significant because they help explain how it has created such a formidable military machine.

First, he was told to do exercises to get him into good physical shape. “The exercises I did in July and August 2014 were physical activity exercises, fitness training, and abdominal exercises,” he says. “After that, I was transferred to a military unit outside Fallujah for a month and then I was sent for a month and a half to Raqqa [in Syria] where I was taught military skills through intensive training courses.

“In Fallujah, I had learned to shoot using Kalashnikov rifles and how to throw grenades. It was a more advanced level of training in Raqqa where I, together with a group of volunteers, learned to use RPG [Rocket Propelled Grenade] launchers and different kinds of machine guns.”

These Iraqi Yazidis fled to Kurdistan when Isis attacked the town of Sinjar (AFP/Getty)

Asked why Isis had taken him and other volunteers to Raqqa for military training, Hamza has an interesting response. “The reason wasn’t because training is not available in Iraq. All kinds of training, equipment and facilities are available in Fallujah, but we were taken to Raqqa to increase our sense of what is called ‘patriotism towards the caliphate lands’ and to introduce us to a new experience and a new revolution.

“When they took us to Raqqa, all the fighters became convinced that the boundary between Syria and Iraq is fake and we are all united under the rule of the Caliphate. Psychologically speaking, I was so relaxed and happy to go there because it was a nice feeling to destroy the borders between two governments and pass through them. This was really a great achievement.”

Executions play an important role in the life of the Isis, not only as a means of intimidating enemies but as an initiation rite and proof of faith by new fighters. Hamza says that in Raqqa trainees like him were sent to watch public executions: “I attended three executions in Raqqa and others in Fallujah. One was of a man believed to be working with the Syrian regime; he was just shot.”

In Fallujah, captured Shia soldiers of the Iraqi Army were executed. “This was the first time that I witnessed a beheading,” he said. “I had been shown some videos made with impressive visual and audio skill. After watching many of these, we were being taken to attend public executions.”

Asked if he had carried out any executions himself, Hamza said that he had not and explained why. “I was not ordered to do so because according to Isis rules, the trainee needs more than six months to be ready to carry out an execution. But this is not the only criterion. The trainee should also show additional skills in his religious education and military tactics as well as many other tests.

“However, the problem was that I was a little bit shaken after attending those executions. I don’t like Shia but when it came to killing them, I was shocked. Although they were showing us videos of Shia militias killing Sunni people, we were troubled when we attended real executions. In November, a large number of Sunni men were taken prisoner on the grounds that they were working with the government...

“In the fourth week of November there were some executions to be carried out. One of our commanders asked me and my fellow fighters to bring our guns to be used in an execution the following day. But the victims were Sunnis, some of whom I knew.

“I couldn’t endure what we were going to do. I tried to explain that, if they were Shia I would do it immediately. The commander said: ‘I will give you another chance later. For now we have Mujahideen [jihadis] to carry out the killing.’”

It may also have been that Hamza had not served the full six months normally required in Isis before becoming a fully fledged executioner.

It was shortly after he had refused to execute Sunni prisoners that Hamza and other Isis volunteers were offered the 13 Yazidi girls for sex. He says that the two events together shattered his idealistic enthusiasm for Isis and created doubts in his mind. He gives a compelling description of his mental turmoil at this time, thinking of “the executions, or more horribly the beheadings, as well as the raping of the non-Muslim girls. These scenes terrified me. I imagined myself being caught up in these shootings, executions, beheadings and raping, if I stayed where I was.”

Now he started to plan his escape, but he knew that this would be difficult and dangerous. He says one Isis fighter had tried to run away but was caught and executed for treason. “The problem is that no one was trustworthy, not even close friends,” he says. Nevertheless, he managed to make arrangements with a friend outside the Caliphate to help him using the instant messaging service Viber, taking advantage of the satellite internet connection that was available to fighters in Fallujah for three hours at a time, three days a week.

Mobile phones evidently worked in at least part of Fallujah (though Isis has blown up mobile masts elsewhere as a security measure), but only some particularly trusted fighters were allowed to have them. “I told my commander that I needed a mobile to talk to my family and he agreed, saying that I will be given more privileges as I prove my loyalty and courage,” Hamza says.

This enabled him to arrange his escape, through friends and smugglers whom he paid to help him. He made his move one early January night when he was put on guard duty on the outskirts of Fallujah, enabling him to slip away easily. It took him five days to reach a place of safety. He has given The Independent details of his itinerary, but publication of these might compromise his security. He is not sure if Isis will pursue him actively and says that he has held back some information about the group because he fears their reaction.

He admits that there are also limits to what he does know: “For example, we, the fighters, were not able to enter what they call the operation rooms, which have many computers and foreign experts, though sometimes my comrades would use the internet nearby and get the Wi-Fi passwords through giving money to the technicians,” he said.

As a recently recruited fighter, he did not meet any senior members of Isis or lieutenants of its leader, Abu Baqr al-Baghdadi. “No, they were always moving from one place to another,” he says. “And they keep talking about al-Baghdadi, saying that he is still living. I am sure and have been told that they [the Isis leadership] are Iraqis only.”

Asked if he thinks Isis will be defeated, he says that this will not be easy, even though coalition air strikes mean “they cannot advance now”.

Hamza says he is now entirely disillusioned with Isis. “At the beginning I thought they were fighting for Allah, but later I discovered they are far from the principles of Islam. I know that some fighters were taking hallucinatory drugs; others were obsessed with sex. As for the raping, and the way different men marry by turn the same woman over a period of time, this is not humane.

“I left them because I was afraid and deeply troubled by this horrible situation. The justice they were calling for when they first arrived in Fallujah turned out to be only words.”