Kiev’s mass anti-government protests are a thing of the past, but the barricades remain, a shrine to the victims. Visitors trickle through the site, paying homage to the Heavenly Hundred, those murdered in the final days of the struggle. The martyrs’ names are taped to the trees, their photographs covered in mounds of flowers. Children holding little Ukrainian flags pose for photographs in front of these monuments. They don’t smile.

They will remember coming here for the rest of their lives, for this is how nations are built: on legends, on emotions, on stories of heroes. Tales of those who stood for months in the square will be told and retold. But that doesn’t mean that the protesters will necessarily have triumphed. On the contrary, Ukrainians are about to learn that the exhilaration of “people power”—mass marches, big demonstrations, songs, and banners—is always an illusion. And sooner or later, the illusion wears off.

This is not to deny the emotional force of the protests. Anyone who has ever attended a rock concert or a football game knows how much fun it is to be part of a roaring crowd. The experience is far more intense when you are standing in a crowd that might change history. Since the eighteenth century, philosophers have tried to describe the hallucinatory power of a mass movement. When Michael Walzer interviewed American civil rights activists they all told him the same thing about protests: “It was like a fever. Everyone wanted to go.” It is precisely because he understands the euphoric power of crowds—and especially because he understands how they can embolden people cowed by an unjust state—that Vladimir Putin, the Russian president, is so determined to prevent Ukraine’s revolution from spreading....