For most of my life I have advocated change in the world’s most closed, repressive nation: North Korea. As a South Korean, my priority has been peace and freedom on the peninsula I call home. But I cannot ignore the erosion of freedom in what was once one of Asia’s freest cities: Hong Kong.

There used to be parallels between South Korea and Hong Kong. Both are economic “tigers,” with basic freedoms and an openness to the world, living in the shadow of authoritarian regimes to the north.

But...