Siddhartha Dhar, who fled Britain last month and joined the Islamic State, poses in Syria with his newborn child. (National News/via ZUMAPRESS.com)

Last month in Syria, Siddhartha Dhar stood in front of a banged-up yellow pickup truck, holding an assault rifle in his right hand and cradling his newborn son with his left.

Dhar’s first four children had been born in London, his native city, but his new baby, wrapped in a fuzzy brown onesie, was born in territory controlled by the Islamic State.

Someone snapped a photo of Dhar, 31, and he proudly tweeted it out as proof that he; his wife, Aisha; and their children had fled Britain and were now living in what the militants consider an Islamic caliphate that will one day reign over the world.

The arrival of the Dhar family in Syria last month represents a key strategic goal of the Islamic State: to build not just an army but a society. The group has vowed to create a nation ruled by Islamic sharia law, and its leaders and online recruiters have encouraged doctors, nurses, lawyers, engineers and accountants to join them in building the institutions of a new holy land.

Entire families — fathers, mothers and children — have answered that call in numbers that have surprised and alarmed analysts who study the extremist group.

Map: Flow of foreign fighters to Syria

“These families believe they are doing the right thing for their children,” said Melanie Smith, a research associate at the King’s College International Center for the Study of Radicalization in London. “They think they are taking them to a kind of utopia.”

Back in London, Dhar’s younger sister, Konika Dhar, 27, said she was heartbroken when she saw the Twitter photo on her phone.

Her brother was now an Islamist militant calling himself Abu Rumaysah, who fled to Syria with his family while he was on bail in Britain after being arrested on terrorism-related charges. In his caustic tweets from Syria, he taunted the U.K.’s “shoddy security system” that had allowed him to jump bail.

But Konika Dhar still thought of her brother as “Sid,” the stylish British kid who gelled his hair, dated girls, listened to Nirvana and Linkin Park, rooted for the Arsenal soccer team, and loved to watch American action movies.

“I think he has actually forgotten Siddhartha Dhar, and he has become this other person,” she said. “I just want my brother to know it doesn’t have to be this way. He really doesn’t have to leave his life. I really miss the children; I can’t imagine not seeing them again.”

Promise of earthly rewards

Unlike al-Qaeda, which operates in many countries but is a stateless army, the Islamic State controls territory that it has taken by force in Iraq and Syria. To create the Islamist society it envisions, the group has gone to great lengths to take over existing schools, hospitals and playgrounds, or to build these and other institutions of daily family life.

“The more they are successful at creating a whole new society, the more they are able to attract entire families,” said Mia Bloom, a professor of security studies at the University of Massachusetts at Lowell who has written extensively about women and terrorism. “It’s almost like the American dream, but the Islamic State’s version of it.”

In the Syrian city of Raqqa, the group’s main stronghold, the extremists have established a clinic for pregnant women run by a female gynecologist trained in Britain. Boys attend school, studying almost exclusively religion, until they are 14, when they are expected to start fighting, Smith said. Girls stay in school until they are 18; their instruction is about the Koran and sharia law, as well as learning how to dress, keep house, cook, clean and care for men, all according to a strict Islamic code.

Bloom said the Islamic State also appeals to women by providing electricity, food and a salary of up to $1,100 per month — a huge sum in Syria — for each fighter’s family. The largesse is funded with money looted from banks, oil smuggling, kidnappings for ransom, and the extortion of truckers and others who cross Islamic State territory.

In Raqqa, once a city of more than 200,000 people, the militants have kicked locals out of their homes and doled out those houses as rewards to fighters and their families, many of whom come from impoverished backgrounds.

“The other jihadi groups promise you all these wonderful things in the afterlife,” Bloom said. “The Islamic State promises to give you stuff in the current life and the afterlife, so you don’t have to wait to enjoy all your rewards.”

Analysts estimate that at least 15,000 people have moved to the Islamic State territories, including several thousand, such as Dhar, from Western countries. While it is impossible to know how many families have joined, Bloom said the majority are probably from Tunisia, Saudi Arabia, Jordan and other Arab nations that have sent the most fighters to Syria.

The United Nations has documented extreme brutality toward women by Islamic State radicals, including reports of women, particularly from minority groups, being stoned to death or sold into prostitution or sex slavery for its fighters.

But the Islamic State uses family imagery in its aggressive and highly polished online recruiting on social media, including videos showing fighters pushing children on swings and passing out toys, and children playing on bouncy castles and bumper cars, riding ponies, and eating pink cotton candy.

Those images are designed to reassure mothers that their children will be safe in a place racked by fighting and regular bombing by the United States and its allies.

However, recent reports from Syria and Iraq suggest that the Islamic State’s propaganda about its public services does not match reality on the ground and that people are enduring painful shortages of electricity, food, medicine and clean water. Smith said she has recently noted increasing complaints from women in the Islamic State territories with whom she communicates on social media.

“To these families, it makes a lot of sense to go there,” Smith said. “They think, ‘This is the path for me; this is my reward.’ But when you think about what it’s really like to live there, it’s unfathomable.”

A death and a conversion

Sid Dhar was 16 when his father died unexpectedly, and his sister said the loss set him adrift.

His parents were Hindus who had immigrated to London from India when they were children, then created a working-class life in Palmers Green on the city’s northern fringe. They lived in a tiny rowhouse on a busy highway alongside other immigrants from India, Pakistan, Greece and Cyprus.

Konika Dhar, a law student, said her family embraced both Hindu and British culture, celebrating Diwali as well as toasting Christmas with Baileys Irish Cream around a decorated tree in their small living room.

“We were just a normal family,” she said, sitting in a coffee shop in their London neighborhood. “Then my dad died.”

When their father died at 46, Konika Dhar said, her mother dealt with her grief privately, while she and her sister turned to each other for comfort. Sid Dhar, the middle child and only male in the house, had no man in the family to lean on, his sister said.

“He felt like he needed to be the man of the house, and make decisions then and there,” she said. “He never asked for help. I felt like him being the only boy in the family, he needed guidance, but nobody was there to give it to him.”

She said his grades started suffering, he became more introverted, and over the next two years he barely managed to finish high school. She said he abandoned his dream of going to college and becoming a dentist, and he took a job as a clerk in a Boots pharmacy.

“I feel like it’s a domino effect,” Konika Dhar said. “One incident has an impact on everything else.”

Dhar sought support from his closest friend, Mizanur Rahman, a Muslim boy from the neighborhood and the son of immigrants from Bangladesh.

They had met when they were 8 years old and called each other “Sid” and “Midge.” They played basketball and video games together. They were born four days apart, and every year on Dhar’s birthday, Rahman called to ask him what it felt like to be older.

Rahman was growing more deeply religious, inspired largely by the sermons of a fiery preacher he met named Omar Bakri Mohammed. Bakri was one of London’s best-known radical Islamist preachers; he had long been linked to al-Qaeda and in 2004 vowed that Muslims would give the West “a 9/11, day after day after day.”

The Syrian-born Bakri was a driving force behind two extremist groups that were eventually banned by the British government: Hizb ut-Tahrir and al-Muhajiroun. Bakri now lives in exile in Lebanon after he was refused reentry into Britain in 2005 because his presence was deemed “not conducive to the public good.”

When he was still a teenager, more than a decade ago, Rahman said he was “amazed” by Bakri’s sermons and persuaded Dhar to come hear him speak at a mosque near their homes. Rahman said Dhar was deeply moved by Bakri’s rhetoric, and the two spoke endlessly about Islam over the coming months.

At home, Konika Dhar watched with sadness as her brother stopped eating his mother’s cooking, because he said he did not want to eat food prepared by a non-Muslim.

He stopped listening to music and watching TV, and he got rid of his bed and slept on the floor, saying he was trying to emulate the simple life of the prophet Muhammad.

One evening in 2002, when Dhar was 19, he and Rahman went to hear another of the many Bakri sermons they attended. On their way home, Dhar walked past his own house and said he wanted to come to Rahman’s place, just a few doors down.

For months Rahman had been urging Dhar to convert to Islam. He said in an interview that Dhar had always found a reason to delay, mainly saying he was worried about what his family would think.

“I was saying, ‘Look, we could all die at any moment, you don’t know when you are going to die, none of us knows the future,’ ” Rahman said, telling Dhar that only Muslims are allowed to join God in paradise. “I told him: ‘If you die now, before you become a Muslim, what are you going to do? There’s no point in delaying.’ ”

They arrived at Rahman’s house at about midnight and sat in the living room.

“Okay, let’s just do it,” Dhar said.

With Rahman and his older brother as the two witnesses required in Islamic tradition, Dhar sat on his friend’s couch and said, in the Arabic he had been practicing: “I testify there is none worthy of worship but Allah, and Muhammad is the messenger of Allah.”

The friends embraced.

With that declaration, he was now a Muslim, and he gave up the name Siddhartha Dhar. He now went by Saif al-Islam, an Arabic name that means “sword of Islam.”

At first, Rahman said, Dhar told no one about his conversion. But two weeks later, he was arrested while participating in a pro-Palestinian protest. Rahman said Dhar’s mother came to pick him up at the police station, and that’s when she learned that her son had converted to Islam.

Marriage, arrest and flight

Dhar wanted to start a family with a conservative Muslim wife, so friends arranged for him to meet a young Pakistani-British woman named Aisha, and they quickly married.

Konika Dhar said Aisha, who is her brother’s age, is a “quite modern woman, but at the same time has very strong views” about her strict interpretation of Islam. She said Aisha covers herself fully in black so that only her eyes are visible.

They were married in a community hall in London on Nov. 9, 2006, when Dhar was 23. They moved to Walthamstow, an East London neighborhood that had been home to a number of radical Islamists, including several of the men convicted in a failed 2006 plot to bomb transatlantic airliners.

Their first child, a daughter named Rumaysah, was born in 2008, and Dhar started calling himself Abu Rumaysah, which means “father of Rumaysah.” Their second child, Usama, was born the following year; Rahman said the name was inspired by Osama bin Laden.

Dhar had wanted his old friend to attend his wedding, but Rahman couldn’t because he was in prison. By then, Rahman and Dhar had been following another radical Islamist leader in London, Anjem Choudary, who was at the forefront of several groups banned by the British government for extremist activities.

In February 2006, Rahman, who had adopted the nom de guerre Abu Baraa, gave a speech about cartoons of the prophet Muhammad that had been published in a Danish newspaper, saying that he wanted to see British troops in Iraq “coming home in body bags.” He was convicted of soliciting murder and served two years in prison.

Dhar was also becoming more and more radical and a close lieutenant of Choudary. Rahman said they were motivated by a belief that the British government, along with Washington, was waging war against Muslims in Britain and around the globe.

With the rise of the Islamic State in the past couple of years, Dhar became one of the group’s most vocal supporters in Britain, giving media interviews in which he called for the establishment of sharia law in Britain.

“Women need to be covered up, men lashed for fornication, and hands cut off for theft and breaking sharia law,” he said in one interview. In another, he said Islamic law forbade him from loving his mother because she was not Muslim.

Dhar was arrested many times on suspicion of belonging to Islamist groups banned in Britain. Most recently, on Sept. 25, Dhar, Rahman, Choudary and seven others were arrested on charges of belonging to a banned group and “encouraging terrorism.” The men were granted bail.

After his release, Dhar and Aisha, who was pregnant, and their four children under 6 years old apparently took a bus to Dover, crossed the English Channel by ferry, then drove to Paris, where they boarded a flight to Turkey.

On Nov. 26, Dhar tweeted the photo of himself in Syria.

In London, Rahman is delighted for his friend and said he has done nothing more than emigrate to a new home.

“He wants to live a better life, in a better place, with real education for his children, with freedoms for himself as a Muslim that he doesn’t have here,” he said. “Ironically, over there, he is far safer than I am here because he is not waiting for the next police raid, he is not waiting for the police to jump on him at any moment and harass him.”

He said that Dhar has no intention of taking up arms to fight and that the Twitter photo with the assault rifle was just a “clever” way to taunt the British authorities. He said Dhar’s most recent job in London was renting out bouncy castles for children’s parties, but he also has experience designing Web sites, so he guessed that he might help the Islamic State with its online presence.

Konika Dhar said her family is devastated by her brother’s choices. “It’s such a shame,” she said. “Nobody said the right thing at the right time, and now look at what has happened.”

She said she hasn’t given up hope that he might return.

“I think my brother has become so involved and consumed in this new movement, he has lost sight of what it is to be a Muslim,” she said. “I want him to be back to normal. He would still be welcome in our family, in our house, with his lovely children.”