CHINESE OFFICIALS and state media were in high dudgeon Monday about minor acts of vandalism following the latest mass demonstration in Hong Kong. Protesters who threw eggs at China’s liaison office and splashed ink on the state emblem “trample on the rule of law,” roared a front-page editorial in the People’s Daily.

Yet Beijing had little to say about the genuinely shocking incident that followed: an assault by an organized group of men on people returning from the demonstration. Journalists, an opposition legislator and bystanders were among at least 45 people savagely beaten with wooden canes and metal rods by thugs dressed in white — apparently to distinguish themselves from black-clad protesters. Police were slow to respond and took no action against the vigilantes.

The high-voltage rhetoric and ugly violence were signs that the regime of Xi Jinping is shedding restraints in its response to the mass movement that has emerged in Hong Kong this summer to defend the city’s autonomy and demand greater freedoms. It means that, more than ever, Hong Kong’s people need the support of established democracies.

Unfortunately, President Trump on Monday sided with the dictatorship. Asked to react to Sunday’s events, the president explicitly endorsed the Xi regime’s behavior. “I think President Xi has reacted very responsibly,” he told reporters. “He’s allowed [the protests] to go on for a long time.”

In essence, Mr. Trump all but offered Mr. Xi an invitation to crack down on a movement that is fighting for liberal values such as free speech and the rule of law. The protests began in reaction to a proposed extradition law that would have undermined Hong Kong’s independent courts and allowed China to pursue its enemies in the city. When the measure was shelved, the marches continued as opposition leaders called for an investigation of police violence.

At the root of the unrest is resistance to what has been steady encroachment by China on Hong Kong’s autonomy, which is supposed to be guaranteed until 2047, and frustration at the regime’s failure to deliver on a promise to allow fully democratic elections in the territory.

As the recently departed U.S. consul general in Hong Kong, Kurt W. Tong, argued in an op-ed published Monday, the United States has a strong interest in how the conflict ends. Close to 1,400 U.S. businesses operating in Hong Kong depend on the preservation of its autonomy. “Chinese leaders,” he wrote, “need to realize they could destroy Hong Kong’s economic specialness if they keep trying to align its political culture with mainland norms.”

Mr. Trump could have made that point. Instead, he noted that he and Mr. Xi were “working on a trade deal” and repeated that the regime had pledged to buy billions more in U.S. goods. Mr. Tong, whose farewell speech was reportedly censored by the State Department to remove language in support of the protesters, wrote in his op-ed that Hong Kong should not be “a token to be exchanged for concessions in trade talks.” As he has now made perfectly clear, Mr. Trump thinks otherwise.

Read more: