I've recently written regarding the unique role of the Meiji Constitution in establishing the Emperor's governance over the national politics of that era. However, it would be innacurate (for me) to suggest that the Japanese did not already possess a long-standing notion of the Emperor's island-wide pre-eminence. The two primary instances of this are Prince Shotoku's "Constitution" of 604 AD and the Taika Reform Edicts of 645-650 AD.
We'll cover here the Taika Reform Edicts (and elsewhere Shotoku's Constitution) which, apparently for the first time, subordinated local governance to the national Emperor.
You'll notice that 645 AD is very early into the Japanese literary timeline. (For example, Japan's unique writing style of hiragana is the much later creation of Kobo Daishi (774 - 835 / aka Kukai).) Thus, in essence, everything written during this era is wholly in the (ancient) Chinese language, using Chinese terminology and vocabulary definitions. And as students of language fully realize, words require concepts. Perhaps primarily for this reason, the Taika Reform Edicts were written under the supervision of Confucian scholars. (During this era, up to the Maoist Revolution, China viewed the Confucian texts as the primary insight for proper (Moral) government).