It helped that Muntasir looked like the leader of a warrior band. Standing at 6ft 7in, muscular and with a flowing beard, he was the heroic father figure the young men who followed him had never had. Interviewed by Khan long after their frenzy had passed, a few of his former followers cited motives that would please the Left. One described how whites would shout “Paki go home” at him “as regularly as the call to prayer”. Racism changes how you think, of course. It has changed an old friend of mine, a secular British Pakistani, who has seen what Islamists do close-up as a reporter in the Middle East and Asia. He’s still an atheist, but says now, “My father was called a ‘coon’, I was called a ‘Paki’ and my children will be called ‘Muslims’.” The name changes, the prejudice remains the same. When I talk to him, he can barely abide criticisms of the Islamism he once denounced because he is so worried about the racism around him.

Another of Khan’s interviewees spoke of how he was angry against society because he was born with a withered arm and leg. A woman said she was angry because she had been sexually abused, but the British police had done nothing. Maybe these aren’t convincing “root causes”, but they would pass muster in some left-wing circles.

Conservatives, meanwhile, could find much to boost them. Muntasir believed the non-Muslim world was Satanic, and the allies of Satan had to be destroyed. He appeared to be justifying every clash-of-civilisations hypothesis.

Yet the rationalisations did not make full or even partial sense. Khan cleverly intercut her interviews with men on the edge of jihadism with clips of two men from Birmingham who were diverted from petty crime, and became better people, by finding Islam. The determinism which holds that Islam must produce a clash of civilisations not only ignores the vast range of contradictory beliefs within a supposedly monolithic religion, and that Shias and Sunnis are currently slaughtering each other across the Middle East, but forgets that Muslims are more likely to be the victims of Islamism than Islamists.

As for my comrades on the Left, if you are a victim of racism, why join a racist and anti-Semitic movement? If you are a victim of sexual abuse, why join a movement that demeans, rapes and enslaves women? Maajid Nawaz, a former member of Hizb ut-Tahrir who rejected theocracy so thoroughly he could soon be the Liberal Democrat candidate for Mayor of London, put it best when he wrote: “Ok far-lefty fellow-travellers of Islamism, I’m a state-school, brown, stabbed-at-by-neo-Nazis, falsely arrested at gunpoint by Essex police, Muslim, divorced, estranged from his child, ex-Islamist, tortured ex-prisoner who’s been mandatorily profiled & DNA’d under schedule 7 at Heathrow airport & blacklisted from countries. I am every grievance you harp on about. And yet your first-world bourgeois brains malfunction because I’m not spewing hate & fitting in your little angry Muslim box. Are you feeling slightly privileged yet?”

Fascism and Communism ought to have taught us that if totalitarian thought systems give men the power and the ability to commit enormous crimes in the name of Utopia they will do it. But it seems as if we have to learn that all over again, just as we must learn to fight all over again.