RUTHFULLY YOURS

On 9/11, Shiraz Maher thought to himself: “Yeah, you Americans deserve this. For meddling in the Arab world. For supporting Israel. You shall reap what you sow, and this is what you’ve sown for a long time.”

Within days the college student would quit alcohol, dump his girlfriend and join Hizbut Tahrir, a radical Islamist group he describes as the “political wing of the global jihad movement.” He quickly climbed the ranks before eventually leaving the U.K. Islamist movement and rededicating his life to countering it.

Mr. Maher is today a senior fellow at the International Center for the Study of Radicalization, King’s College London, where he researches Europe’s homegrown Islamist movement and profiles the droves of young Britons who are decamping for Syria and Iraq to wage jihad with ISIS, aka the Islamic State of Iraq and al-Sham.

These include Abdel-Majed Abdel Bary, a wannabe rapper from a posh west-London neighborhood who recently posted a Twitter selfie of himself holding a severed head. “Chillin’ with my homie,” read the caption, “or what’s left of him.” Abdel Bary is also suspected to be the terrorist who addresses the camera before beheading American journalist James Foley in a widely circulated online video, though Mr. Maher thinks the masked figure is a different British jihadist.

Abdel Bary is one of 500 to 600 British citizens who have joined the Islamic State, and Mr. Maher’s center estimates about 2,200 foreign fighters from Europe are operating in the region. “Globally we believe the number to be somewhere in excess of 12,000. We’ve counted 74 different nationalities that are represented on the ground.”

Many fighters have European passports, which means they can travel around the Continent and even enter the U.S. with relative ease. Two-hundred-fifty fighters have already returned to the U.K., according to Mr. Maher.

Not all of the foreigners in the region initially intended to join ISIS, which is only one of several groups fighting Bashar Assad’s regime. Yet in recent months the Islamic State has emerged as the most successful and prestigious outfit, while recruits to the other groups have slowed to a trickle.

ISIS proved appealing in part because it was the easiest group to join. Says Mr. Maher: “We know of a lot of people including Britons who’ve tried to join Jabhat al Nusra”—al Qaeda’s Syrian franchise—”who were turned away because Jabhat felt it didn’t know them and so couldn’t trust them. And then they went to ISIS, and ISIS welcomed them with open arms.”

Battlefield prowess was another advantage. “ISIS has been particularly successful at bringing in fighters from Bosnia and Chechnya,” Mr. Maher says. “The greatest human asset that an army can have is fighters with combat experience. And the Bosnians and Chechnyans of course have huge experience, a great deal of sophistication and knowledge about how to fight guerrilla warfare.”

Cultivating a brand helped, too. “ISIS developed a strong social-media presence,” Mr. Maher says, while “other organizations didn’t have the same glamour. And we’re dealing with young men. They want to be with a strong horse, with a winning team. At the moment, ISIS has momentum.”

Finally, the Islamic State has a veneer of authenticity. Its leader, Abu Bakr al-Baghdadi, presides over “land that these guys regard as pure and holy,” Mr. Maher says. “There’s a lot of stuff in normative Islamic theology which talks about bilad al-Sham, the land of Syria. The Hadiths, the prophetic tradition, say that when God sends angels, they rest in Syria after their journey.”

Reverence for this angelic pit stop hasn’t stopped the Islamic State from turning it into hell on earth. “In the last 10 months we’ve seen British fighters serve as suicide bombers,” Mr. Maher says. “We’ve documented British fighters executing prisoners of war. And we have documentary evidence of British fighters torturing people in their care.”

The typical British Islamic State terrorist is male, in his 20s and from a South Asian background. “He usually has some university education and a history of Muslim activism,” Mr. Maher adds. The fighters broadly fall into three personality types.

The first is the adventure-seeker. “They’re in jihadist summer school or camp,” Mr. Maher says. “I’m with my buddies, we’re hanging out and we have these great weapons—AK-47s, RPGs.” The adventure-seekers are often involved with U.K. gangs or drugs, and they might consult “Islam for Dummies” before traveling to Syria. They publish photos of themselves eating fast food, swimming and playing soccer in al-Sham. The message they telegraph to friends back home is: “We live better lives here than we were in London—come.”

Then there are the “really nasty guys,” Mr. Maher says, “the ones who will show off a severed head on Facebook and say, ‘Yeah, I just beheaded this son of a bitch.'” These guys, Mr. Maher adds, “should definitely never come back.'”

The third type are “what you might call idealistic or humanitarian jihadists for want of a better phrase,” Mr. Maher says. “They would say, ‘Look, haven’t you seen what’s happened to the women and children of Aleppo?’ ” Over time, they become hardened and no longer mention the innocents they came to rescue. “The land belongs to Allah,” they now say. “We’re here to impose Islam.”

Mr. Maher himself fits the third type most closely, and had he been born a decade later he might not be sitting across from me at a restaurant eating steak tartare and sipping Guinness. “If I were younger and instead of 9/11 it was the Syrian conflict,” he says, “there’s a very, very good chance I would go. Instead of studying them, I would be the one being studied.”

Shiraz Maher was born in 1981 in Birmingham to British-Pakistani parents. When he was still an infant, his father’s accountancy practice took the family to Saudi Arabia. “I never had a concern about what kind of society Saudi Arabia was,” he says. “We lived in a Western compound, with everything you could want: tennis courts, swimming pools, cricket, basketball, bike races, all gender-mixed.”

Yet the political atmosphere in Saudi Arabia became more tense after the first Gulf War. When he was 11, he owned a Daffy Duck T-shirt with the slogan “I Support Operation Desert Storm.” One day an ordinary Saudi asked why he’d wear such a shirt. “I said, ‘Why not? Saddam’s a terrible man.’ The man said: ‘No. This is an American conspiracy. These people use us as an excuse to establish bases on holy soil.’ “

In 1995, at age 14, Mr. Maher moved back to the U.K., and five years later he enrolled at Leeds University, in northern England. Then came 9/11, an event that he says “activated” latent anti-American ideas he’d imbibed while growing up in Saudi Arabia. By the time the U.S. invaded Afghanistan, he had recovered his Muslim faith, changed apartments so he could live across the street from the local mosque, and joined Hizbut Tahrir.

Hizbut Tahrir—Arabic for “Party of Liberation”—campaigns for a global Islamic state but advocates a broad, political path to the caliphate. “It’s not anti-violence per se,” Mr. Maher says. “It applauds suicide bombers but believes suicide bombing is not a long-term solution.”

Mr. Maher says the U.K. government for years looked the other way as a generation of British Muslims was radicalized. “In the late 1980s, early ’90s,” he says, “this country opened its doors to radical Islamist preachers from around the world who began to preach a very hard-line, totalitarian message about what Islam should look like. That message has always been a minority view,” he says, but it is persistent.

Hizbut Tahrir, for example, organized a 1994 conference in London about the need to establish a caliphate. The event drew Islamists from Sudan to Pakistan, yet Mr. Maher says U.K. law enforcers took a blasé attitude: “These exotic guys with beards are talking about a new state. OK.” The result was that the “idea of having an Islamic state had been normalized within the Muslim discourse,” Mr. Maher says, and young Muslims were taught to think of their British identity as something “filthy.”

Government missteps continued even after 9/11. The 2003 “Prevent” counterterror strategy, as Mr. Maher describes it, involved “empowering fairly radical people, like Abu Hamza, who were saying to people: ‘Don’t blow anything up here, go abroad and do it. That’s fine.’ ” Abu Hamza, an Egyptian imam who for years led London’s notorious Finsbury Park Mosque, currently awaits sentencing in the U.S. on terrorism charges.

Today, Mr. Maher says, London is much more aware of the need for the “ideology of Islamism to be tackled.” In 2005 when he began to have doubts about Hizbut Tahrir, Mr. Maher was alone and without support. He’d risen from a cell leader to a regional director and even been invited to join the group’s U.K. executive committee. Yet during graduate study at Cambridge, Mr. Maher encountered more pluralistic strands of Islam and came to conclude that Hizbut Tahrir’s radical ideology “will lead to terrorism. It’s also basically rubbish.”

He left the group on July 7, 2005—the day the London Underground bombings killed 52 people and maimed more than 700. The bombers were from Leeds. They weren’t Hizbut Tahrir members but belonged to the same radical milieu. “Were we, was I, part of the flame that warmed up the anger?” he asks. “Absolutely. I don’t go around feeling guilty, but we contributed to the momentum of hatred and anger.”

Does his own journey from Islamist to anti-Islamist give Mr. Maher hope? On a positive note, secular dissidents, moderates and Muslim liberals have found a voice in the West and in the Middle East. Thanks in part to his own efforts, the British branch of Hizbut Tahrir has been decimated. The group tells its members that the “party is your umbilical cord to Islam,” Mr. Maher says, and young Muslims having second thoughts need confidence: “Tell them: ‘You’ve been in a cult. There’s a world outside.’ ” Hizbut Tahrir rallies used to draw 20,000 supporters. Today “they struggle to get 1,000.”

But those gains are overshadowed by breathtaking jihadist advances in Syria and Iraq. Save for a small minority of idealistic Islamic State members who question the group’s brutality and long to come home, Mr. Maher says that “most of these guys have to be fought. Militarily we have to confront them, and when I say ‘we’ I mean the United States and Britain.”

It didn’t have to be this way. “Bashar Assad is one monster,” he says. “Had we gone in and taken him out, there would have been other monsters but not at this level. The jihadists needed this crisis. They needed the power vacuum.

“Did bin Laden win? Yes. He did not want there to be a strong hand in the region for the world’s greatest and most powerful force for good—the United States. And voluntarily we chose to disengage, and watched as these radical millenarians came in and took over.” He knocks on our table for emphasis: “This is a disgrace and a humiliation.”

Mr. Ahmari is a Journal editorial-page writer based in London.