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"This takes things to another level."

So says a Parisian who was at the Stade de France last night, one of six sites targeted by at least eight terrorists who managed to kill 127 people using a devastating mix of automatic weapons and suicide bombs. President Francois Hollande blamed the attack on ISIS, which is also suspected of involvement in the downing of a Russian plane over the Sinai Peninsula in late October. This means that in the space of two weeks the group has dramatically raised its profile from a regional cancer to an international menace, a development that may have consequences for the West’s approach to the volatility that has engulfed areas of the Middle East. 

And coming after an attack earlier this year on the offices of Charlie Hebdo, the latest violence, which appears to have been conducted by jihadis, is likely to worsen Europe’s fraught relationship with the continent’s Muslim communities, whether that manifests itself in concerns about homegrown terrorism or the flood of refugees pouring in from Syria and elsewhere. Far-right political parties like France’s National Front are already on the rise, and this will only play into their hands.

Hollande said France will be “merciless toward the barbarians” of ISIS, which he declared had launched an “act of war” with “internal help.” The extent of that mercilessness is the question that will occupy France and the Western world in the days and weeks to come.