The good old "they were here FIRST" bit.
Every single country in the entire WORLD has the "original inhabitants" and the "we came and took it over" inhabitants. . . that is just a part of life and history and how things change.
Even now things are still done that way - territories in the Middle East keep changing hands (for one example).
In 200 years from now can an original inhabitant from one country that *use* to be part of another country really demand a different level of consideration because when their ancestors lived there that area was ruled by someone else?
Even Mexico and other countries in South America have strict rules and regulations that they enforce when it comes to illegal-citizenry and squatters.
That whole argument is aged like a skunky beer.