Question:
[[ But when does it end? ]]
Answer:
Actually, it "ends" when a nation is established in which is culturally, ethnically, and (horror of horrors!) racially homogenous.
Was not that whole point of "nations" to begin with?
Actually what you describe isn't a nation, it is third world, backward tribalism. Ideal nations (first world successful nations) are neither homogenous (racially, ethnic identical) nor heterogeneous (multicultural), they are Assimilationist, compromising those bringing the best of their culture, ethnicism, etc, all creating a new, unified and unique culture.
Multiculturalists are pushing toward heterogeneous society of groups infighting among themselves because they don't unify under common values.
Racists and their sort push towards homogenous society which is a stagnate, tribal society that results in both physical and cultural inbreeding.
The best parts of the US are Assimilationist united under a common value of individualism and liberty.
Think Greek Hellenization, the US Melting Pot, the Catholic Church (as a universal church/political entity), or even to an extent, Roman Imperialism.