Why is Japan a Nation-state?

Answer 1

The nation state is a state that self-identifies as deriving its political legitimacy from serving as a sovereign entity for a nation as a unit. The state is a political and geopolitical entity; the nation is a cultural and/or ethnic entity. The term "nation state" implies that the two geographically coincide. Nation state formation took place at different times in different parts of the earth but has become the dominant form of state organization

Japan is also traditionally seen as an example of a nation state and the largest of the nation states, with population in excess of 120 million. It should be noted that Japan has a small number of minorities such as Ryūkyū peoples, Koreans and Chinese, and on the northern island of Hokkaidō, the indigenous Ainu minority. However, they are either numerically insignificant (Ainu), their difference is not as pronounced (though Ryukyuan culture is closely related to Japanese culture, it is nonetheless distinctive in that it historically received much more influence from China and has separate political and nonpolitical and religious traditions) or well assimilated (Zainichi population is collapsing due to assimilation/naturalization).