I'd think that one advantage of nationalism is the unity it builds among people who share the same nation, who start to think of each other as co-citizens. One disadvantage of nationalism is the hate can be directed against outsiders.
Nationalism is a political, social, and economic ideology that is based on the promotion of the interests of a certain nation. Nationalism alleges that each nation should be able to govern itself and be free from any external interference from foreign powers.
An advantage of nationalism is that it aims to preserve a nation's culture and traditions. Things that many countries are losing nowadays as a product of globalization. It was also the driving force behind independentist movements in many regions throughout the world.
A disadvantage of Nationalism is that, combined with religious beliefs or ethnical backgrounds, may lead to prejudice towards other populations. This is particularly dangerous as it could be a leading cause of war.
