A nationalist is someone who holds the well-being of their nation as a high-priority thing. This does not necessarily make a nationalist an enemy of other nations: nationalists are usually preoccupied with the well-being of their own nation, and just want other nations peacefully mind their business.

This is how a nationalist can strongly oppose a nation's current government, when the government is not caring for the nation's prosperity (in the nationalist's eyes).

Most liberation movements across the world were and are openly nationalist.