Above all it must be Isil. The leaders of that death cult may be mad, but they are far from stupid. I suspect that they are already looking beyond the territory it controls today to the World Caliphate from which all non-believers will have been cleansed. It seems likely to me that their next target may well be Europe, rich, decadent, full of non-believers. If Europe could be destabilised, its non-believers driven out or killed, its churches, shrines and monuments destroyed, then the World Caliphate would be brought much nearer.