The current concept of a nation state is greatly helped by nationalism, and giving people a passport and a nationality at birth does help to have them connect to the highest level of government and not just their local region, but people have died in the name of their country long before anyone wrote a word about nationalism. People have an inner need to connect to others, the people in charge of a village want their village to be part of something greater, people in charger of that greater thing do too, and on and on.

Brainwashing people with nationalism can certainly be helpful for people with power, i don't disagree, but I don't think it on it's own is all that is needed for countries to come into being and people to rally behind it's flag. Pride is another thing we are all born with in greater or lesser extend. We want do not only want to be part of a tribe, we want our tribe to be the best tribe.