CHINA’S COMMUNIST leadership appears convinced it can handle opposition in Hong Kong by the same means it has recently employed on the mainland: brute and uncompromising repression. For several years, unrest in the territory has been growing because of Beijing’s refusal to deliver on the promise it made when the former British colony reverted to Chinese rule, which was that its executive would be chosen by popular suffrage. On Sunday came part of the regime’s response: A Beijing-controlled Hong Kong assembly of just 1,200 delegates installed a hard-line bureaucrat as chief executive, ignoring the fact that polls showed she was less popular than the other major candidate allowed on the ballot.

The next day, the Hong Kong proxies of Xi Jinping added injury to insult: Police informed nine activists who led mass protests in 2014 to demand a truly free election that they were being charged — after all this time — with crimes carrying multiple-year prison sentences. It was a naked crackdown on dissent in a city where freedom of speech and assembly is supposed to be guaranteed under the “one country, two systems” autonomy Hong Kong was promised — and a signal that the pro-democracy movement will no longer be tolerated.

There’s a good chance that the repression will backfire, as it has repeatedly before. The 2014 “umbrella” protests, which paralyzed the center of Hong Kong for 75 days, came in response to a plan for elections that failed to deliver on the promise of universal suffrage. The official who oversaw it, Carrie Lam, is the executive imposed by Beijing on Sunday.

Since the Umbrella Movement was suppressed, Beijing has refused to compromise with the moderate opposition while repeatedly violating the territory’s autonomy, including through the lawless abduction of book publishers and a businessman. As a result, sentiment has been growing for complete separation from China. Several young activists with pro-independence leanings were among the six opposition candidates elected last fall to the local legislature. The regime responded by preventing two from taking their seats and is now moving against the other four.

Whether or not new mass protests erupt, the continuing hard line is likely to drive more support to the opposition, including those favoring independence. The prospect that China could persuade Taiwan to accept reunification under a “two systems” formula will become ever more remote. And investors will question whether Hong Kong can continue to offer the stability and rule of law that have been the foundations of its prosperity.

One price Mr. Xi probably won’t pay, however, is damage to relations with the United States. Defending democratic freedoms in Hong Kong has been a long-standing and natural part of U.S. foreign policy. But Beijing appears to be calculating that the Trump administration will shrug off the assault on the democrats. So far, that’s the case. The State Department offered no criticism of the arrests on Tuesday, saying only that it was “aware” of press reports about them. On Monday the U.S. consulate in Hong Kong actually congratulated Ms. Lam on her “victory in Sunday’s balloting.” The reality that her selection was a travesty that betrayed the promise of democracy was simply ignored.