Chinese foreign policy under Xi Jinping rests on certain basic assumptions: that in a just world, China should be hegemonic in East Asia, the center of a system in which the other regional powers pay their respect and take direction from China, as was the case for two millennia prior to the 19th century; that regions once considered by Beijing to have been part of China should be “reunified” with it; and that a revived China should have at least an equal say in setting the norms and rules of international life. These goals are achievable, Mr. Xi asserts, because...