Written by Vivian Wang Wang Xiaodong once gave a speech declaring that “China’s forward march is unstoppable.” He published essays calling on China to build up its military. He co-wrote a book, bluntly titled “China Is Unhappy,” in which he said the country should aim to control more land and shape global politics. “We should lead this world,” he said. Now Wang, a 66-year-old Beijing-based writer once called the standard-bearer of Chinese nationalism, has another message: That nationalism has gone too far. For years, it was Wang whom many Chinese dismissed as too radical, as he railed that the Chinese...