Protesters in Hong Kong have delivered the most stunning rebuke to Chinese tyranny since the Tiananmen Square uprising of 1989. The question now, after eight weeks of demonstrations, is whether China’s dictator, President Xi Jinping, will respond with the same brute military force used to crush that democracy movement 30 years ago. Serious observers worry the backlash is coming.

For Mr. Xi, who took power in 2013, the situation in Hong Kong presents an immediate threat to his domestic political legitimacy. State repression,...