The devastating attack in Nice, carried out by a Tunisian man living in France, came at a moment of political ferment in Europe and seems likely to give even more fuel to anti-immigrant, anti-Muslim movements that are challenging established parties across the region.

The third major attack in France in 17 months — a murderous truck ride by a disturbed loner claimed by the Islamic State as a "soldier" in its war on the West — was used by far-right movements and anti-immigration, populist parties within the political mainstream as fodder for their arguments that Islam poses a mortal danger to European societies. Curbing immigration, a rallying call for Britons who voted last month to leave the European Union, as well as for many supporters of Donald J. Trump in the United States, is the answer, they suggested.

"This is a war," Geert Wilders, the Dutch right-winger who is gaining in his country's polls, said on Twitter on Friday. "And it will not stop until we close our borders for Islam and de-Islamize our societies. No more terror. No more Islam!"

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