https://archive.boston.com/news/globe/ideas/articles/2007/08/05/the_downside_of_diversity/
And if you want a contemporary example, look at what's happening in modern American politics. We're becoming more divided because we no longer feel like a single people anymore.
Most adults know what multiracialism is already, but here goes.
Multiracialism is the idea or practice of a society being somewhat equally composed of various races. I'll agree that no country is 100% any single race. However, America was an ethnostate for most of its history in that we favored white immigrants in order to keep America as white as possible. And this worked very well, since even during the fifties, the white population didn't fall below 85%.
In 1965, it was officially decided by the government that we were no longer going to favor white immigrants, but we were promised this new move wasn't going to change America all that much. Clearly, we were lied to.