What are the disadvantages and disadvantages of nationalism?

I personally think nationalism is a diesease. It makes ppl self serving and along with propoganda, arrogant. It causes others to suffer while you sit in a comfy chair. It stops advancement in any type of world peace. It causes wars, it destroys less fortunate countries.

on the bright side, you get a "rah rah" feeling when your soldiers kill ppl. and I guess, it is good for moral.