When you go to war and win, guess what it's yours

Why any nation has a say is ridiculous.

Au contraire. If asked, both nations fighting over a piece of land will say it's theirs. The maps change once some other countries agree on which line in the sand to recognize. Defending and maintaining territory can look a lot like ongoing occupation, you feel me?

And anyways, borders are intangible things (and are subject to change), but people and their homes and their cities aren't. Forcing these people to get the hell out may be "the way things go", but that doesn't mean it's chill. It also doesn't have to be something we support. Not for nothing, ruining/ending the lives of Real Actual Humans for the sake of an abstraction is one of the least savory human traditions, and I'd personally be fine if we left it in the past.