Consider the specific issue at the heart of the Hebdo nightmare: the question of whether European nation-states can successfully integrate Muslim immigrants, and what will happen if they don’t.

Here France looks like the crucial test case. It has the largest Muslim population of any major European country, and parts of that population are more assimilated and others far more radicalized (16 percent of French citizens expressed support for the Islamic State in a poll last summer) than elsewhere on the Continent.

Not surprisingly, the response to Islam is divided as well: Muslims are regarded more favorably in France than elsewhere in Western Europe, and yet French politics features an increasingly potent far-right party, Marine Le Pen’s National Front, whose electoral clout is now likely to increase. Meanwhile, France’s foreign policy has distinctive (often military) entanglements across Northern Africa and the Levant, which means the ripples from French domestic politics have more room to spread and then return.

So if there’s a path to greater Muslim assimilation and inclusion, it’s more likely to be pioneered in France. If Islamic radicalism is going to gain ground or mutate into something more pervasive and dangerous, it’s also more likely to happen in France’s sphere of influence than elsewhere. And if Europe’s much-feared far right is going to complete its journey from the fringe to the mainstream, it will probably happen first in Paris.

French politics is likewise central to the fate of the wider European Union project, which is in crisis at the moment because of the gulf between Germany’s interests and the interests of the E.U. periphery, Greece and Italy and Spain. But that gulf (and the weight of 20th-century history) means that the Germans, however economically dominant, cannot hold the union together on their own. Instead it’s France, for reasons of history and culture as well as geography, that has to bridge the divide between Europe’s north and south and make the E.U. work politically. Unless, of course, the French gradually and fatefully choose not to, in which case the entire project will fall apart or be completely reconceived.