They conquered it, they moved out or suppressed the indigenous barbarians, and they are productively using the land in furtherance of civilization.
I can't speak for Nebraska, but out in my part of the country (Pennsylvania), the land was legitimately bought from the Indians for a fair price in a settlement made without war. My right to my property in this state is based upon real title, not force. The force enforces my title. It does not create it.
So, West of the Mississippi and North of Rio Bravo, what?
We give it all back?