the idea that a culture or people deserve their own autonomous state (the nation) and that the good of the nation equals the good of the people. furthermore that nations are in conflict with each other and that mixing makes them inferior. goes hand in hand with glorifying national identity.
Historically it has been used as a geopolitical tool to build nations (Germany, Italy), expand them (Great Britain) or gain liberty from foreign rulers (most of the global south, the balkans).
Latest Answers