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.