China’s president, Xi Jinping, declared back in 2014 in a series of speeches delivered in private to officials that he intended to crack down harshly in Xinjiang, the north-western region of China where about 13 million Uyghurs and other Turkic Muslims make up half the population. The reality of that “strike hard against violent extremism” campaign, which followed decades of repressive policies, is now clear: Chinese authorities are committing...