An estimated 900 Britons travelled from the UK to join the murderous Islamic State.
Terrifyingly, as their caliphate crumbles, at least 260 of them are still unaccounted for.
The biggest nightmare for the security services is trying to keep tabs on battle-hardened maniacs.
Some 180 with UK passports were killed on the battlefield either by allied warplanes or in fighting on the ground.
An estimated 450 of them are believed to have returned to Britain, where they are kept under as close scrutiny as the security services can manage.
Only a handful have been prosecuted, mainly because of a lack of evidence.