Qionglin New Village sits deep in the Himalayas, just three miles from a region where a heavy military buildup and confrontations between Chinese and Indian troops have brought fears of a border war. The land was once an empty valley, more than 10,000 feet above the sea, traversed only by local hunters. Then Chinese officials built Qionglin, a village of cookie-cutter homes and finely paved roads, and paid people to move there from other settlements. China’s leader, Xi Jinping, calls such people “border guardians.” Qionglin’s villagers are essentially sentries on the front line of China’s claim to Arunachal Pradesh, India’s...