U.S. is the only country in the world that is a Nation (or would I be right in calling it a country?) by "Invitation" - and where the Americans are about 0.1% of the population, I mean those whose forefathers were born in North America with roots there. Some Dutch people wanted to emulate this example and added their own brutal twist in South Africa that resulted in the worst carnage Humanity ever faced (& will not be repeated I hope). While the World has to go through lot of convolutions in dealing with South Africa & setting it right, the former has gone Scot-free.
The rest in U.S. (don't ask the opinion of KKK or some hardened WASPs) were all 'Invited' from Europe predominantly, robbed from Africa and allowed from Asia. But for some cultural nuances, stemming from Attitudes that are insignificant, one from the British Isles finds USA no different, unlike the Chinese, the Indians (I have plenty of relations who are Americans who flock here like birds in Christmas season), the Tibetans (they aren't separate but are called Chinese, I suppose), Thais etc each of who live in two worlds.
-Peru : A world hewn out by he Spanish, who erased the ethnic differences like waves on Atlantic Ocean, every Peruvian wears his ethnicity in his blood. They are an ethnicity that is a "wounded civilisation " (a VS Naipaul's expression).
-Thailand: Largely ethnic sameness, but for their over-Americanisation that passes off as development & affected accent of Anglo (American?) speech.
-Romania: Romania is single-ethnic (like single-malt), in spite of attempts at Americanisation of their culture.
-Tanzania: Being underdeveloped, development is seen here as the Golden Arch (McDonald), Coke or Pizza huts. the natives (those who can afford) have all but given up their traditional food to the underprivileged & disadvantaged of their countrymen whom Mzee Nyerere wanted to uplift; and in the process added to the expense of imported, (elegantly, but unaffordably) packed food & fruits, not grown locally. As per a survey done a quarter century ago, Tanzania amongst the developing world is the "most ethnically diverse' country, composed of a variety of African tribes (whom the rest of the world see as one ethnicity). I have a feeling that all African countries by reverting back to their traditional (non-wheat based) cereals & grains can increase the prosperity of their peoples. Facing the Indian Ocean in the east, Tanzania acquired sizable Arab ethnic component (mostly in Zanzibar & Pemba).