The success of the military campaign against Isis in Syria and Iraq has left behind a diplomatic and legal problem: what to do with the British citizens who travelled to join and fight with Isis, but who have survived hostilities. The problem has been brought to a head by the capture, by a group of Syrian Kurds, of El Shafee Elsheikh and Alexanda Kotey — two Londoners who were members of ‘the Beatles’, a group which tortured and beheaded at least 27 hostages.