I recently met up with two young men at a quiet café in Beijing and told them I wanted to talk about June 4, 1989. One of them, let’s call him Wang, slapped the table and rose from his seat. “Sorry, I’m leaving!” he announced.

A moment later, though, he sat back down: He was just joking. “This is the reaction most people have,” said Liu, his friend. “Their thinking is, ‘Better not mention it.’”

“Another reaction is, ‘What is that?’” Wang said. “A lot of people don’t know about this.”

Twenty-five years after June 4, 1989, even China’s educated youth have only a foggy understanding of the incident, and they’re skittish about discussing it openly. Textbooks don’t mention the violence that left hundreds, maybe thousands, dead in the streets of Beijing. The Chinese Internet has been scrubbed of all but the official accounts. (The first result on the search engine Baidu is a short article from People’s Dailyconcluding that the incident “taught the party and the people a useful lesson.”) The Chinese government has arrested dozens of people in recent weeks for planning or participating in events related to the anniversary, and police have warned foreign journalists not to cover the story. Still, most young Chinese people I approached were willing to talk—as long as they could remain anonymous...