An extensive buildup of barriers along China’s 3,000-mile southern border is under way, according to public documents, official statements and interviews with residents, ostensibly to battle Covid-19 but with likely long-lasting ramifications on trade and travel.

The small Chinese city of Ruili, in the far south next to Myanmar, has seen a major construction project in the past two years. It is a border fence equipped with barbed wire, surveillance cameras and sensors.