The most difficult problem faced by the authorities is to obtain information about potential jihadis, who can find their way to Raqqa in Syria, IS's headquarters, after flying, seemingly innocuously as tourists, to Germany or Turkey. Police need to elicit information from friends and relatives, which is not often forthcoming. A "misprision of terrorist felony" law, requiring notification to police by any who have reasonable cause to believe that a person is planning to go abroad to fight for a proscribed terrorist group might provide some incentive, especially to family members who fear that they will never see their jihadist-minded relatives again, other than behind bars.

Law cannot change minds already made up by religious brainwashing or tempted by the promise of an eternal paradise. It does, however, set standards of what is right and just, and in the case of crimes against humanity it identifies and stigmatises the conduct up with which a civilised society can never put. The crime, after all, is "against humanity" because the very fact that a human being can conceive and commit it demeans us all, as members of the same human race. We owe it to the world to prosecute all of "our" IS fighters that we can get our hands onto turn them in, rather than turn them away.