The more urgent concern, however, should be on Europe, and how the jihadist movement reconstitutes itself inside the continents borders after the Islamic State. The principal concern for security services has been the threat posed by Isis returnees, and with good reason. There is nothing new about Europeans travelling to jihadist conflict zones, but more travelled to Syria and Iraq for jihad than every previous jihadist insurgency combined. Not all made it home, but many did or will in future.
Available evidence does suggest that only a minority of foreign fighter returnees attack at home, but this should not necessarily be a cause for complacency. The statistics alone do not account for the constant ideological transformation and evolution of militant Islamism in the West: when commuters were murdered in London in 2005, or when Mohammed Merah executed Jewish schoolchildren and soldiers in France in 2012, these acts sparked internal debates on legitimacy within salafi-jihadist circles. Today, thanks to the work of salafi-jihadist scholars, there would be no such debate.
Nor do low rates of attack tell the whole story of the impact of returnees. Each generation of returnees from conflict overseas has successfully helped to cultivate a new and larger generation of extremists. Isis hotspots in Europe often directly overlap with recruitment hotspots for jihadist insurgencies from decades ago. If the Isis generation is able to socialise a new, more violent, more extreme generation of salafi-jihadists then it may be years, or even decades, until we feel the full force of the most recent returnee wave.
