Inevitably, that raises the memory of Tiananmen. But that was 30 years ago, before China grew into an economic powerhouse with global reach, capable of sustaining a trade war with the United States and of dismissing criticism from Britain, Hong Kong’s former colonial master, with a disdainful shrug: “We urge Britain to know its place.” Many China watchers believe that the new China has worked too hard shaping its image as a tough but responsible power to lose it by bringing Hong Kong forcefully to heel.

Of course, China’s hard-line leader, Xi Jinping, must be seething. One of the central government’s most ardently held tenets is “One China,” which stipulates that territories ceded in weaker times remain sovereign Chinese lands that will eventually revert to full Chinese control. Hong Kong was first; the far greater prize is Taiwan. Whatever hopes Beijing may harbor of bringing Taiwan, a democracy, into its fold would become even more remote if Hong Kong’s “one country, two systems” formula disintegrated in a Tiananmen-style crackdown.

Focused ideologically on “One China,” mainland China seems incapable of understanding what the Hong Kong protesters really want. Though as a police state China no doubt has reams of information from officials, spies and informers, the rulers probably hear what they want to hear, that most Hong Kongers are content with their relative prosperity and freedom and will eventually reject rabble-rousers. So the protests must be the work of outside agitators seeking a “color revolution” like those that swept through Ukraine and Georgia.

But the Hong Kong protesters are not waging a color revolution. Their goal is not some abstract ideal of democracy, but the memory of a way of life they have no intention of letting Beijing take away. They are fired not by foreign agents but by China’s attempts to deny them full participation in their government, which they were promised through 2047 in the agreement under which Britain ceded control over Hong Kong.

That’s a just cause, even if the protesters have at times resorted to violence. In the past, the United States would be expected to intercede as the world’s greatest champion of human rights. Alas, President Trump has so far treated the Hong Kong protests largely as a diversion from his trade war with China, at one point telling Mr. Xi that the United States would not interfere in what China did in Hong Kong. Mr. Trump has since issued a variety of conflicting tweets and remarks, of which the most supportive of the pro-democracy demonstrators was a demand that Mr. Xi “work humanely with Hong Kong.”