After a Chinese surveillance balloon flying over the US earlier this year provoked an international incident, open-source intelligence analysts went blimp hunting. Some zeroed in on an enormous hangar in a remote part of Xinjiang, which looked like the infrastructure needed to deploy some kind of lighter-than-air vehicle. This collection of buildings and roads has been carefully watched for more than a decade. The complex, operated by the People’s Liberation Army (PLA), is thought by intelligence analysts to house research and development centers for laser weapons capable of attacking US satellites, as well as test facilities for ballistic missiles, electromagnetic...