Most countries today _are_ based on ethnicities. They may not purely be of a particular ethnicity, but their basis is some ethnic grouping. There's a reason why all Germans speak German, and all Finns speak Finnish (to name a couple).
This is one of the things that's so great about America. It wasn't founded on an ethnicity; but an idea. But if you go to some of the other (more backward) countries, people still think of American as an ethnicity.
It's a not-insignificant marker of nationality for nations which have experienced diasporas. The citizenship laws of both Israel and the Republic of Ireland grant citizenship on a fairly liberal basis, based upon Jewish or Irish descent.
Pre 1940s Europe is a transnational jumble of overlapping ethnic groups. Result: war after war after war.
Post 40s and 50s after millions of people relocate to their linguistic and ethnic homelands and coalesce into new nations carved out of empires: lasting peace. Last major war was in former Yugoslavia, which failed to get with the ethnic nationalist program.
Hell, even Belgium is falling apart now. You are dead wrong on this. Ethnic nationalism is the only system that works, long term.
You conveniently left out the late 1930s and 1940s, in which a nation-state perpetuated ethnic war and genocide before being stopped primarily by two fairly transnational states.
Would you be interested in a summary of 20th century armed conflicts, classified by the type of states involved? There are a couple in Asia and Africa I'm thinking of.
