Luckily it is, essentialy, the basis of customary international law, otherwise the entire world would disintegrate in to ethnic violence until we ended up with

Luckily it is, essentialy, the basis of customary international law, otherwise the entire world would disintegrate in to ethnic violence until we ended up with perfectly aligned ethno-states rather than the borders we have now, the majority of which are in place as the relics of failed empires.