“Would you like some nice ISIS fighters?”, US president Donald Trump asked French president Emmanuel Macron on the sidelines of the NATO leaders meeting in London this month. “I can give them to you”. Macron didn´t think it was funny. “Come on, let’s be serious”, he replied. The awkward exchange between the two leaders was emblematic of the wider question at hand; should countries take back their foreign terrorist fighters? And if so, what would that mean for their national security?