President Xi has become, since his inauguration in 2012, China’s most powerful leader since Mao. His likely longevity in the post is strongly suggested by the presence of those around him in senior positions—all older and therefore unlikely to succeed him. He has been personally popular and has published his own thoughts on China and communism in a form also not seen since the time of Mao. Xi-ism is not quite a thing yet, but the signs are there. So why, then, is his authority under threat? Let’s start with Hong Kong. The stream of protests, which have run there over the last six months—and their enormous scale—has disturbed Beijing. The protests, whatever their immediate cause, are protests against the mainland government and hence the Communist Party headed by Xi. Diplomatic pressure from the U.S., in particular, has hurt. The ineptness of the Hong Kong SAR government, with knee-jerk and ill-thought-out responses that have frustrated both pro- and anti-establishment groups, has not helped. The mainland liaison office in Hong Kong—effectively an embassy—has a new head, who …