Socialist Bernie Sanders’s emergence as a front-runner for the Democratic presidential nomination is puzzling to many of us who have experienced socialism. We thought it dead—good riddance—when the Soviet empire collapsed 30 years ago. Our mistake.

One can only assume that the hardships endured by many millions of Russians, Chinese, Africans and Europeans have been forgotten, or never learned, by a new generation of Democratic voters. Mr. Sanders says he is a “democratic socialist,” whereas those unfortunates of the past...