The ongoing military offensive against ISIS’ largest city, Mosul, is seen in Iraq as the end-all, be-all goal of their protracted military offensive, a chance to deal a major defeat to ISIS, and to potentially end their status as a major holder of territory inside Iraq.
There are potential downsides though, and European Union officials are seeing some big ones, as the fall of Mosul back to the Iraqi government would mean a major shakeup of ISIS fighters and figures. Some would go to Syria, undoubtedly, but many might also return to their countries of origin, which includes a lot of EU nations.
It isn’t that the EU didn’t foresee ISIS returnees as a problem. Indeed, they’ve been worried heavily about it for years. But the Mosul fight could mean a lot more returnees a lot sooner than anyone figured. The EU has struggled mightily with attacks carried out by the very limited number of ISIS figures in western Europe now, and their security isn’t ready to handle a many-fold increase in those numbers.
As fighting with ISIS cuts back on their territorial holdings, foreign recruits will have less reason to remain within the “caliphate” and more reason to return to their countries of origin, with a bunch of new training and deep contacts to international terror.