I'm Australian, but my ancestors came from England, Croatia, Ireland, Scotland, France and I think Germany and somewhere else (I'll have to check my family tree). On my mum's side my family's been here since the late 1700s (they were convicts), and I haven't got any Aboriginal Australian ancestors.

Which ancestors would I have to choose? haha

The ancestors of most Americans came from Europe or Africa. I think they'd face the same delemna.

Whether or not I'd fight, and who I'd fight for, depends entirely on what the war is about. If it's just countries fighting because they're different countries (WWI), then I wouldn't fight at all.

It depends on whether or not you are a citizen of that country. Citizenship comes with responsibility. If you are going to swear an oath to become a citizen, then I believe you should defend that country. If you would not fight for that country, then you probably should not become a citizen. 

Katty,

I am guessing that you are a young person, so you may not know about a  similar situation during World War II here in the United States.

When the United States and country X (an Asian country) went to war against each other in 1941, there were about 100,000 people here in California who were born in country X or whose parents had been born in country X.

The American government thought that maybe some of those 100,000 people (many of whom were American citizens) would try to help country X.  So all of those 100,000 people were put into special prisons.

I  will fight for justice.

  I will stock up  on Marshmallows and Peanut Butter.