Hong Kong’s protests started more than five months ago to defend the rule of law, and on Tuesday China underscored what’s at stake. The comrades in Bejiing threatened to overrule a Hong Kong court in violation of China’s legal promises.

The territory’s High Court on Monday struck down an ordinance banning the face masks that protesters have used to shield their identities from police. The two judges who heard the case ruled that the law violated Hong Kong’s Basic Law, or mini-constitution, because it had been implemented by...