Two takeaways from Sunday’s European Union elections: First, the centrists—the moderate right and left—were decimated. For the first time since 1979, Christian Democrats and Social Democrats, the reliably pro-European bloc, no longer hold the majority in the 751-member European Parliament. Second, the far right—the Europe bashers and nationalists—scored big, increasing their take to about 170 seats. In Britain, the Brexit Party trounced both Labour and the Tories. In France, Marine Le Pen’s National Rally outpolled President Emmanuel Macron’s party and its allies.

The...