In an article for The Daily Yomiuri, Naoki Matsumoto of Waseda University raises the question to his fellow countrymen of ‘When did we become Japanese?’ and goes on to answer his question with reference to the myths of the Kojiki (712) and Nihon shoki (720).
His thesis is that the Yamato polity integrated the myths and gods of those they conquered, much in the manner of ancient Rome. It’s hardly an original idea, but it does lead him to suggest that the ceding of Izumo by Ohonamochi to the Yamato led to the people there becoming ‘Japanese’. When exactly the ‘kuniyuzuri‘ took place is a matter of dispute, though it’s thought to be sometime in the fourth century, possibly around 380. Matsumoto appears to be saying this was Japan’s 1066 moment when the country as we know it had its beginning.
On the left is Inasa no Obama, or the Little Beach of Inasa (photo by Matsumoto). Ohonamochi aka Ookuninushi-no-Mikoto allegedly swore to cede control of his kingdom at this place, just near the Izumo Taisha shrine in Shimane Prefecture.
For the full and somewhat lengthy article, see here.