The world is learning some unpleasant truths about China’s Communist Party amid the Covid-19 pandemic, and the latest example comes from Hong Kong. Police on Saturday arrested more than a dozen of the city’s most prominent proponents of democracy on what has all the earmarks of orders from Beijing.

The arrested include Martin Lee, 81, a main author of Hong Kong’s Basic Law who is considered the father of Hong Kong’s democracy movement; 71-year-old media tycoon and our contributor Jimmy Lai; at least eight former opposition...