More than a country, I'd say the Japanese are a nation, see meaning #4 of the definitions below. I think America (the US) is nation only in sense of meanings #1, 2 and 3 which are synonyms of country.

country kŭn′trē noun 1) A nation or state. 2) The territory of a nation or state; land. 3) The people of a nation or state; populace. 4) The land of a person's birth or citizenship. 5) A region, territory, or large tract of land distinguishable by features of topography, biology, or culture. 6) An area or expanse outside cities and towns; a rural area. 7) The people of a district who are eligible for jury service.

nation nā′shən noun 1) A relatively large group of people organized under a single, usually independent government; a country. 2) The territory occupied by such a group of people. 3) The government of a sovereign state. 4) A people who share common customs, origins, history, and frequently language; a nationality. 5) A federation or tribe, especially one composed of Native Americans. 6) The territory occupied by such a federation or tribe.