London Mayor Boris Johnson's controversial call to arrest anyone returning from Syria and Iraq made headlines this week.
But it's also put the spotlight on the difficulty European governments have in trying to stop the recruitment of hundreds of their citizens to the ISIS cause.
In a column for The Telegraph this weekend, Johnson wrote, "The law needs a swift and minor change so that there is a 'rebuttable presumption' that all those visiting war areas without notifying the authorities have done so for a terrorist purpose."
Needless to say Johnson's column — written in the wake of James Foley's execution by a militant with a British accent — wasn't without its critics.
LIBBY WIENER, ITV NEWS CORRESPONDENT: "Guilty before proven innocent. The former Attorney General Dominic Grieve dismissed that as draconian. Labor were also unimpressed."
MEMBER OF PARLIAMENT CHUKA UMUNNA: "I don't think these are sophisticated, sensible or responsible comments by the mayor of London."
Human rights advocates also criticized Johnson.
JAMES WELCH, LEGAL DIRECTOR OF LIBERTY ADVOCACY GROUP: "People might be aid workers, part of the aid effort there, they might have family in the region that they're visiting. You can't just presume that people who are going to those countries are going to be involved in jihadist activities."
Most outlets report estimates of between 400 and 500 British nationals fighting in Iraq and Syria under the ISIS flag. (Video via Voice of America)
There are a number of reasons why they're going, but, as Newsweek reports, ISIS' use of social media has been a driving force. "Islamic State have so far run a very successful campaign of using public forums such as Twitter, Facebook and YouTube, whilst avoiding detection by authorities."
As well as heavily produced promotional material.
ATIKA SHUBERT, CNN CORRESPONDENT: "The most recent slick propaganda production from ISIS, designed to show their planning operation and showcasing video taken from a drone camera."
Still, as The New York Times points out, experts say there is also a small group of radical imams in the U.K. who "preach a brand of Islamic supremacy ... that demonizes non-Muslims and gays and justifies militant acts." As one analyst put it, “You don't go online looking for a pair of shoes to buy and suddenly end up becoming a jihadi.”
So far, anti-extremist bodies in the U.K. have focused on working with local mosques, with groups like Quilliam using panels and speakers to spark dialogue about extremism.
And a former counterterrorism chief interviewed by The Guardian said a cautious approach would be more effective and practical than Boris Johnson's suggestions and said cracking down could infringe on the right to free speech for groups that don't "obviously and directly incite to violence."
Interestingly, researchers at King's College in London estimate the majority of Western European fighters in Syria are actually French, and French officials say as many as 800 insurgents have taken part in the fighting, twice the number of British fighters. (Video via Press TV)
Yet the bulk of recent coverage from major outlets seems to have focused on British fighters, rather than the French, possibly because of all the attention drawn by the fighter with the British accent in the Foley execution video.
The King's College researchers estimate nearly 2,000 Western Europeans have so far joined the fighting in Syria.