Things Nationalism has solved:
- Tribalism: Instead of small bands of blood related people fighting against other small bands of blood related people in battles that are waged purely for genetic reasons - nationalism instead allows individuals unrelated by blood or marriage to live and work together.
- Nomadic Life Style: Instead of having to wander and completely live off the land nationalism allows unrelated people to build civilizations (agriculture, governments, academic institutes, i-phones, etc).
- Ceremony and Identity: Nationalism promotes a sense of community by common practices and beliefs while also providing individuals a clear understanding of their place within their communities and within their broader worlds.
. . .
Nationalism - in and of its self - isn't necessarily a bad thing. And quite often it's pros far outweigh it's cons. However, it can be dangerous when used as an "us vs. them" pretext (which, unfortunately, does happen).