“The numbers of this generation are astonishing,” says Richard Barrett, a former head of counterterrorism for MI6 and now a senior vice president of the Soufan Group in New York — which, among other things, provides strategic security and intelligence services to multinational organizations and government. “We’re talking about some 20,000 fighters from about 90 countries around the world — which could be three times more nationalities from which today’s fighters are being drawn. And although the number of fighters is fairly comparable, those who went to Afghanistan arrived over a period of 10 years; in Iraq and Syria, it’s been only three, four years.” He went on: “There’s also more of a common purpose today and thus more camaraderie. A great majority of today’s fighters are fighting for the Ummah — the community of Muslims — which they see as being under threat. With Afghanistan, the battle was against the Soviets, which was much more opaque. Also, and this is very important, in the case of Afghanistan, many of the jihadists were being sent by their governments; it was state sponsorship. Today we are seeing 20,000 young men from around the world — aided by social media, to a great extent — simply packing their bags and taking the initiative.”

The counterterrorism bill was being harshly criticized by human rights groups for, among other things: the power to temporarily ban re-entry to the country, for two years, of any British citizen suspected of fighting in Syria or Iraq; to empower airport security officials and border guards to confiscate the passports of any suspected would-be jihadists before they set off; and to require Internet-service providers to retain communications data from private accounts. The government defended its position on national-security grounds. “One of our major problems here is that, until now, we have had no significant border checks for exiting the U.K.,” a member of Parliament, who was not authorized to speak on the subject and so asked not to be named, explained. “As a result, we have no idea where someone has been when he or she comes back. As far as the exclusion provision is concerned, anyone who is not permitted to re-enter the country has the right to appeal, and that appeal must be granted, as long as that person agrees to an interview with a member of the security services. The point of this section is to stop individuals suspected of terrorism from coming back without making their return clear and obvious to the government.”

I asked Barrett what he thought of the bill’s provisions. He replied: “This bill was driven by two factors: the government’s responsibility to protect the state, of course, but also domestic opinion and, as part of that, commensurate pressure on the government. This is the government’s way of saying: ‘We’re tough on terrorism.’ But a negative in all of this was the government’s speed in drafting the bill, with elections coming up.” What he had hoped the bill would do, he said, was to establish ways of reintegrating those who had gone to Syria or Iraq. “Before this bill was passed, according to official figures, 260 Britons had already returned, and only 40 or so were arrested, and may be brought to trial. So we’re talking about some 220 people who came back and got on with their lives. Now, with this new legislation, if someone really wants to come back because he’s disenchanted with all the infighting, with all that he’s seen, what’s going to happen to him? This is not something you can legislate with a blanket approach. What do you do? Tell this young man, You’re going to have to stay there for two more years, and thus become even more radicalized?”

He paused and then said: “Another thing which is terribly important is how to stop the flow; how to dissuade young men — and some women, too — from going out at all. And who is better to do this than those who have returned? Let them tell potential recruits: ‘It’s terrible! Don’t go!’ If you want to send a message, you’ve got to choose who that messenger will be. If you want to get credible messengers, you’ve got to find people with credibility.”

Aldgate East is the last stop on the District line before London proper ends. It is a welter of shops and coffeehouses and traffic jams; noise and people are everywhere. It used to be a largely Jewish neighborhood, and then in the 1970s and 1980s, waves of Bengalis, from the Indian subcontinent, arrived, and the neighborhood was transformed. It is now home to the largest mosque in Britain, the East London Mosque, a massive, sprawling structure set somewhat incongruously next to — and surrounding three sides of — the tiny Fieldgate synagogue.

I had come here to meet Moazzam Begg, an Islamic activist, a prolific writer and speaker and a former prisoner at Guantánamo Bay who, along with 15 other Britons who were also detainees, sued the British government for complicity in their detention, successfully. More recently, Begg, who now works with a human rights and advocacy group, had traveled twice to Syria since the carnage there began.