In Hong Kong, hundreds of thousands have taken to the streets to protest a bill permitting the arbitrary arrest and extradition of citizens to mainland China. Hong Kongers were assured in 1997 that they would enjoy a “high degree” of autonomy when the British transferred the colony back to China. That “high degree” is dropping lower and lower.

Five hundred miles northeast, in Taipei, the Taiwanese are watching the dispute closely. For them it foreshadows what “reunification” with China would look like. And for the U.S., it’s...