A century ago this week, communism took over the Russian empire, the world’s largest state at the time. Leftist movements of various sorts had been common in European politics long before the revolution of Oct. 25, 1917 (which became Nov. 7 in the reformed Russian calendar), but Vladimir Lenin and his Bolsheviks were different. They were not merely fanatical in their convictions but flexible in their tactics—and fortunate in their opponents.

Communism entered history as a ferocious yet idealistic condemnation of capitalism,...