Nationalism, good or bad?

most of the time i would like to think that i have a good, open, and tolerant mind to other people's opinions, believes, and views of things,
in short; i try to understand the minds, views, and opinions of others.
but that also results in my thoughts clashing together on a regular basis;

this time the spawn of this caos, are the two different arguments;

"Nationalism is a good thing."

"Nationalism is a bad thing."

now, i'm gonna spare you guys the TL;DR of "my opinion" part (like you guys weren't just gonna skip it anyways.),
because right now my mind is going crazy with layer upon, layer upon layer, ect. of arguments topping arguments.

so here is the question:
is Nationalism good? or bad?

Nationality is a method of categorization, Nationalism is a way of thought. If you're asking people's opinions on the former, you're an idiot. If the latter, you should probably be clearer in your post.

If we were to remove all countries and make the Earth a single country nothing would change (in the scope of this discussion). Instead of it being "nationalism" it would be "regionalism". We already have that now.

Sports team belonging to Town A wins national championship. Some fan's of that team believe their town is the best thing in the world as a result. Rivalry with runner-up Town B ensues. The scale is much smaller and a bit different but the idea is the same.

It's both good and bad. Good because people have an excuse for a common interest on a bigger than this town scale. Bad because.......pick a war.

Everything's good in moderation. Too much nationalism will cause ethnocentrism. Too little nationalism causes little national unity.

It all comes down to the simple fact that unites people everywhere, everyone thinks they are better then everyone else.