As Russian troops have poured into Ukraine, officials in Beijing have fumed at any suggestion that they are betraying a core principle of Chinese foreign policy — that sovereignty is sacrosanct — in order to shield Moscow. They will not even call it an invasion. “Russia’s operation” is one preferred description. The “current situation” is another. And China’s leader, Xi Jinping, says his position on the crisis is perfectly coherent. “The abrupt changes in the eastern regions of Ukraine have been drawing the close attention of the international community,” Mr.…