LONDON » Born and raised in leafy West London, Ibrahim Ahmed always supported the local soccer club and listened to what he called "white music." But in school he was a "Muslim," and he became increasingly disaffected from British society. When recruiters approached him in a mosque 18 years ago and told him that he could fight a holy war right here at home, he readily agreed.

In Sweden, Robert Orell was reading "Mein Kampf" and preparing for his own war. The immigrants who had bullied him at his school were now, in his view, bullying his culture as liberal politicians stood by. He fantasized about bursting into Parliament with one of the guns his neo-Nazi friends had hidden in the woods.

The ideologies that once motivated Ahmed and Orell could hardly be more different. Yet strip away ideology and what emerges are two strikingly similar tales of radicalization, militancy and, in the case of these two men, deradicalization.

Both had grievances that eroded their self-esteem and made them angry. Both were seduced by a narrative that put them at the center of a greater cause and offered them what they craved most: a sense of belonging and a plan to act on their resentment.

Both eventually walked away from violence, dissuaded not by law enforcement officials or relatives but by former extremists like themselves.

The parallels are instructive as Europe tries to recover from two deadly attacks in two months, both of them committed in the name of Islam. Religious ideology plays a central role in the radicalization of young Muslim Europeans being lured to join the Islamic State or kill in the group’s name at home. But the psychological process underlying radicalization is remarkably universal, terrorism experts say.

"We are so beguiled with ideology, we miss the fact that jihadis and neo-Nazis have a lot in common," said John Horgan, the author of "The Psychology of Terrorism" and director of the Center for Terrorism and Security Studies at the University of Massachusetts at Lowell. "The similarities of how they get engaged, involved and disengaged in terrorism by far exceed the differences."

Europe’s long and checkered history of far-right extremism and other varieties of militancy, from violent Marxism to the Irish Republican Army, makes the Continent a rich laboratory for counterextremism and deradicalization.

Today, the recruitment success of groups like the Islamic State is considered the greatest threat. But decades of researching, infiltrating and countering other movements offer some lessons at a time when governments are scrambling for ways to head off the threat beyond tightening security, analysts said.

One lesson, they said, is that former extremists have a central role to play in the argument against radical temptations. They have a credibility that governments lack.

"We need to replace fantasy with reality," said Amy Thornton of the Department of Crime and Security Science at University College London. "Formers play a very important role. Only they can credibly say: Syria is not a video game, you may end up cleaning toilets, babysitting on the front line; it’s not what you’re being promised."

Another lesson, experts say, is that debunking extremist propaganda alone is not enough. Outreach efforts are most effective, they said, when they offer a counternarrative and tangible alternatives to violence.

One pioneering program in Denmark treats onetime fighters not as potential terrorists but as wayward youths. Closely watched by the authorities around Europe, the program involves counseling, help with readmission to school and meetings with parents. Although now being applied to Islamic radicals returning from the Middle East, it was first developed in 2007 for far-right extremists.

There are limits to the willingness of governments to rely on such a program. But experts in radicalization said that understanding the process by which people fell for the medieval brutality of a religious ideology is vital to combating it.

"We won’t make any progress at all if we continue to obsess over the question ‘why’ someone becomes an extremist," Horgan said. "A better starting point is asking ‘how.’"

For Orell, now 34, it started on a summer evening in Stockholm in 1995.

Then an anxious 14-year-old with divorced parents and difficulties in school, he suffered regular intimidation by a gang of boys from immigrant backgrounds. The only place he felt safe was with a youth club, where he discovered punk rock with lyrics that spoke of Viking conquest.

That evening, Orell wore a Viking T-shirt and a pendant of the hammer of the Nordic god Thor. Two older boys, far-right recruiters, handed him a sticker with a Viking wielding a sword. The caption read: "Stand up for Sweden."

Years later, he drifted into a group of soccer hooligans with links to neo-Nazis and eventually into the neo-Nazi scene itself. He dropped out of school and moved in with other extremists. He read anti-Semitic pamphlets and wore black outfits. Every weekend, he and his friends would prey on nonwhite youths, badly beating them up.

"Race was my religion," Orell said. "I was fighting a holy war."

Holy war was also what was proposed to Ahmed in a South London mosque in 1997.

He did not grow up religious. His parents, shop owners who had immigrated from Pakistan and India, raised him and his two brothers in a middle-class neighborhood where they were the only nonwhite children. At school, white boys threw racist insults and chipped slate tiles at him.

"Often it was 15 of them against three of us," Ahmed said. When he joined a Muslim gang, it was to defend himself, but also to take revenge.

At a mosque one day, he met men who told him Britain was a Dar al-Harb, a land of war, and that he was a soldier. Within a month, he had joined the security wing of Hizb ut-Tahrir, an international Islamic organization committed to establishing a caliphate in the Middle East.

For two years he was on call as part of a secret Muslim brigade that went after anyone reported to have "given a Muslim brother or sister grief," he said. He carried a gun and threw Molotov cocktails.

Did he ever kill anyone?

He paused. "I honestly don’t know."

Eventually, both men began having doubts.

Orell, who was trying to live up to Aryan ideals by quitting alcohol and drugs and working out daily, was put off by less disciplined comrades. When a group of Neo-Nazis was arrested in connection with the murders of two Swedish police officers, Orell was appalled. It was around that time that he started talking to a former militant who had moved to the countryside with his family.

"It was good talking to someone without being judged," Orell said. "I was still every bit as radical, but I was getting disillusioned with the group."

The friend introduced him to Exit, a charity offering far-right extremists support as they left the movement. Many social workers at Exit were former extremists, too. They listened, played soccer with him and gradually "chipped away at the black and white."

In Britain, Ahmed planned to fight in Bosnia. He had never paid attention when his family said that Islam and violence were incompatible. But when a Salafi preacher who had once been involved in gang violence told him as much in 1999, he listened.

"He said he shared my grievances but that violence was not the way to address them," Ahmed said. "He said ‘I get it, I’ve been there.’"

That is the message he tries to get across to the teenagers he counsels, like a 16-year-old boy who is tempted to go to Syria.

"I don’t judge him," Ahmed said. If he were 16 today, he added, he might be tempted to go to Syria himself.

Both he and Orell, who now runs Exit, say counterextremism work has become trickier over the years. The Internet has given militants direct access to teenagers. The video-game culture glorifies extreme violence. And radical movements have become smarter at marketing.

Ahmed tries to channel Muslim discontents away from violence. "I ask them: ‘When was the last time you wrote to your MP?’" he said, referring to a member of Parliament. "Have you ever run a fundraising campaign? Written a letter to your local newspaper?"

Indeed, the post-extremist lives of Ahmed and Orell seem to indicate that radicalism need not be destiny. They know one another through a network of former extremists brought together by the Institute for Strategic Dialogue, a charity active in combating extremism.

"If we had met 15 years ago we probably would have killed each other," Ahmed said. "Now Robert is a friend."

© 2015 The New York Times Company