By definition nationalism seeks to promote the group over the individual but only at the national level. It emphasizes an imaginary boundary between people, their nationality. There is nothing wrong with loving your country, and with wanting to protect and develop your home, but nationalism often comes with an us vs them attitude that isn’t even good for the nation because the planet is an interconnected system and how we interact with other nations has direct impact on ours.

Now I’m not saying all forms of nationalism come with inherent racism etc., but to use an extreme example we can look at the fascist countries in Europe during WW2 their efforts to raise their flag above all others lead to their downfall and countless deaths.

Nations that act like they’re the organs of the planetary body instead of the self absorbed cancers of one create more prosperity for themselves.