Armed Bolsheviks seized the Winter Palace in Petrograd—now St. Petersburg—100 years ago this week and arrested ministers of Russia’s provisional government. They set in motion a chain of events that would kill millions and inflict a near-fatal wound on Western civilization.

The revolutionaries’ capture of train stations, post offices and telegraphs took place as the city slept and resembled a changing of the guard. But when residents of the Russian capital awoke, they found they were living in a different universe.

Although...