Ong, you do realize that essentially all populated borders are populated with at least large pluralities who have more in common with those across the border than the aggregate population of their country? Under the paradigm you propose, pretty much any militarily superior country could gerrymander border regions of neighboring countries and claim the territory as their own.

Further, if you were a resident in Donbas that has been flooded with little green men from Russia, are you really going to feel free to express your desire to remain part of Ukraine?