I searched on google but i dont found a decent answer. Google sucks.
I wanted to know: If a person has dual nationality, for example, Brazil and United States, and both go to war, but I live in Brazil but I was born in the USA, to which country should I follow?

Probably the one that you want to follow, dont base your opinions on who you should support on where you were born/live.

In practical terms, unless you plan on moving back to the country you were born in, you should probably support the one you are living in

If both enforce mandatory draft, you'll probably be drafted in the country you live in.

Pick the country that's going to win

Like me Im Mexican but live here. Americans hate me but I live here and I support it.

Even if someone is American they don't necessarily follow USA when they go to war. Actually it's pretty common for Americans to be against USA...

If it was a war where both countries were attacking each other, pretty soon you would develop animosity towards the country which is dropping bombs on yours. In the end which is more important to you, random people far away dying or people close by that you know dying. Most people end up going for their country of residence in an all out war.

When USA goes to war usually it invades, so people have the luxury to choose whether to be for or against the war since it doesn't really affect them.

None. If you're not sure which country you want to win, there is no reason for you to risk your life.
I mean first you risk your life and then even when country that you supported wins you're not fully satisfied with that.

Thanks guys.
I'll close.