Clearly, deradicalization can work—several academics currently working in the field are former extremists, and for more than a decade, one of Britain’s leading counter-extremism programs was run by a former al-Qaeda sympathizer who had traveled to Afghanistan. Was there any obvious difference between those who were successfully deradicalized and the others, I asked Snell. “A lot of them are very bright,” he said. They got interested in new ideas, learned new information—and their stark worldview began to crumble. Others, though, were unable to understand the moral choices they had made or the effects of their actions. “Not every individual can be changed,” Ali said. “Just like we can’t stop every terrorist attack … and that’s the conflict. We want to maintain that open society. There will always be individuals we can’t reach.”

Perhaps the best way to think of it is that every terrorist sympathizer who reaches prison already represents a failure—a demonstration of society’s inability to stop extremist ideas from flourishing. Snell acknowledges that the Prevent program, which was designed to tackle extremism, instead left many British Muslims feeling that their whole community was being stigmatized. It also initially failed to address the growing threat of far-right terrorism, and it tried to recruit unrelated public-sector workers, such as teachers, to spot potential extremists. But, Snell said, it did force the government to recognize the root causes—the isolation, the lack of role models, the foreign-policy narratives—that contribute to extremism.