I have to work right now so can't work through all the examples listed by another poster below, but will try to come to this thread later. But if you look at e.g. historical Europe or Rome you will rarely find borders defined by ethnicity, and you will rarely even find people defining themselves by a singular ethnicity. Look at the e.g. Franks, the Germanic political entity that ended up defining modern France (and Holland and Germany and Belgium, etc.) A Germanic-speaking tribe that took over a Celtic (Gaulic) and Latin speaking territory, went to war with other "Germans", switched to speaking Latin in some places, but not all, and on the territory where they once ruled you now have a shattered mirror of dozens of ethnicities ("German", "Dutch", "Walloon", "French", "Alsatian", etc) in a nested fractal pattern depending on what lense you use, none of which correspond to any exact political border and which are fluid and changing from decade to decade.
My take: Ethnicity exists, is fluid, etc. Ethnonationalism is a tool, an ideological tool, mobilizing ethnicity for its own ends, using a specifically modern era mechanism ("nations", which are only ~200-300 years old) for the purposes of whoever is wielding it. E.g. the ambitions of an e.g. Milosevic type personality.
My ancestors are from both Alsace and Germany, so I take an interest in this topic.
