Nationalism by definition involves suppressing a segment of the population (usually opposed to the unifying party and/or allied with external parties).

It's ugly but true.

It also happened in the United States. In Britian. In France. In Germany. In Spain. In Italy. In ancient Greece and Rome.

There are lighter / harsher ways of going about it, though the appropriateness of those is also defined by the opposition's actions.

Successfully creating a national identity, where there were disparate groups before, breaks a lot of people in the transition.