[State-sponsored] Shrine Shinto affirmed a national quintessence for the Japanese—a spiritual essence even more fundamental than any "religion." To criticize this essence was itself proof that one was not of the "body [or essence] of our (Japanese) state," the literal meaning of kokutai [国体]. Originally emphasized by the Mito school thinkers, kokutai became a favored term among political thinkers during the foreign war years. The term referred ... to both the empire and the emperor. Loyalty to the emperor was not a choice but a recognition and expression of one's own Japaneseness....SOURCE: Shinto: The Way Home, by Thomas P. Kasulis (U. Hawai‘i Press, 2004), pp. 139-140
It has become commonplace, incidentally, to translate kokutai into English as "national polity," but this rendering is linguistically and philosophically inappropriate. Linguistically one can say in English that virtually every civilized country has some kind of "national polity," but in Japanese kokutai applies specifically to the purportedly unique form of Japan's political/spiritual/imperial structure. In fact, technically speaking, kokutai is not a political structure at all but a metaphysical ideology legitimating a certain form of polity. Furthermore, it is linguistically peculiar to translate kokutai as "polity" when the word "polity" is not translated into Japanese as kokutai unless it is applying specifically and uniquely to Japan. And philosophically there is the history of the idea of polity itself in the West going back through medieval thought to the ancient Greeks. In this intellectual development, the Western assumption has usually been that people or societies fashion their particular polity. The Japanese idea, by contrast, is that the organization is a sacred unit going back to the time of creation, a unit that resonates affectively, intellectually, and uniquely in the soul of every Japanese. In this respect, to translate kokutai simply as "national polity" is to despiritualize, obscure, and defang the distinctive force of the Japanese term.
I've wondered about this from the time I took my first Japanese history class in high school. I don't think I've ever heard the term "polity" used except in the Japanese context.