Walking across the White House lawn a few weeks ago on his way to an event in Ohio, President Trump was asked by a reporter about the protests in Hong Kong. They have been constant since June, when authorities were about to pass a law that would have allowed Hong Kong residents and even foreign nationals to be extradited to the mainland for prosecution under China’s legal system.

“Hong Kong is a part of China,” Mr. Trump said. “They’ll have to deal with that themselves.”

This...