Yeah nah it’s theft. It doesn’t matter how it’s framed. In the middle ages and before, where what side of the border you lived on was pretty much irrelevant to your life, it’d make sense to think of it the way you are. But in the modern day, where taking land often means completely altering and destroying the lives of the people that live on it, forcibly removing it from them is theft. The implications that come from it being theft is a completely different conversation.