It was a threat Europe’s security services had long feared coming true. In June, Mehdi Nemmouche, a French-born jihadist who had returned to Europe after fighting in Syria with the Islamic State in Iraq and Syria, shot four people to death in an attack at the Jewish museum in Brussels. While the Brussels attack was the first successful one executed by European jihadists returning from Syria, there have been other numerous other planned assaults.

Last fall, Britain’s MI5 disrupted an attack allegedly planned by a cell of “returnee” jihadists for a Mumbai-style mass attack on civilians in London. And in March of this year, French counterterrorism authorities announced they had foiled an imminent attack by a jihadist from Syria who was preparing to strike in southern France. Moreover, several European countries have broken up networks recruiting European Muslims to fight with jihadist groups in Syria, most recently in Spain where police arrested eight people suspected of forming a group that recruited militants to fight in Iraq and Syria.