In a ploy familiar to students of one-party dictatorships, Xi Jinping has used the pretext of an anti-corruption campaign to purge supporters of his predecessors, Hu Jintao and Jiang Zemin. This, more than anything else, makes it clear to their respective factions — and to everyone else in China, for that matter — that their one-time patrons no longer have the power to protect them. Astonishingly, even Mr. Jiang’s own sons have fallen victim…