I mean if people want to have a distinct culture or ethnicity they will or will not as they desire.
If anything the history of the nation state is forcing 12 different ethnic groups into a single culture that only reflects part of their heritage. Look at Nazi Germany trying to suppress Bavarian culture as insufficiently German. Or the UK suppressing Welsh and Irish culture.
Nation States enforce a mono culture to build false unity, always has been.
>from competing states
Ukraine Iran and Afghanistan are really the only states that have done defense "Alone" in recent memory. Everyone else joins larger groups for defense. NATO for example. The Nation State isnt the headline defensive organism, and when it is, its generally a failure.
>Nations organize themselves into states to protect and promote their interests.
This is the closest to the truth, except it would be better put as
>Nations organize themselves into states to protect and promote the interests of the ruling party.
Always has been, ever since the 2 dominant forms of government were Palace Economies and Temple Economies. The goal was suppression and slavery of farmers to ensure the quality of life of the people in the cities, so they can build secondary and tertiary industries to enrich the Prince or Priest at the top of the pack.