URUMQI, China—This city on China’s Central Asia frontier may be one of the most closely surveilled places on earth. Security checkpoints with identification scanners guard the train station and roads in and out of town. Facial scanners track comings and goings at hotels, shopping malls and banks. Police use hand-held devices to search smartphones for encrypted chat apps, politically charged videos and other suspect content. To fill up with gas, drivers must first swipe their ID cards and stare into a camera. China’s efforts to snuff out a violent separatist movement by some members of the predominantly Muslim Uighur ethnic...