A nation often starts out as a family/group of families which expand across a land. They then either assimilate into larger groups or overpower smaller groups. Migrating clans that settle among this nation usually assimilate or usurp the ruling class of the people's lands they come to. Once groups of people become stabilised in a region their governance usually centralises & through intermarriage, a distinct phenotype of that nation develops. A culture, language & sometimes regional dialects form depending on how centralised or decentralized the culture of the people is.
This has usually been the case throughout human history unless a form of separation is law. Then, because of forced separation, two or more clashing identities form & in time these identity groups form their own countries or one identity rises to power across the whole land & crushes the other groups.
A people of the same land often become a unified nation, in time despite having many cultures - provided that the unification is not forced. When unity is forced, a peaceful union is less likely to be. When there is no common language or culture & there are clashing views on what the land should become, peaceful unity is also less likely &, at times, separation & dividing the land into self-governing cultural, linguistic or ethnic regions becomes a sensible option.
The concept of national identity is fluid because people assimilate & learn new languages or adopt neighbouring cultures which causes an unforced & natural unity in a region. People decide themselves what is the reason for their nation's unity - whether it be ethnicity, religious/cultural practices, riches etc. The default reason for national unity among the most prosperous nations in the modern world is usually ethnicity.
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