Relying on borders to keep peace reminds me of relying on laws to keep people away from alcohol and cannabis.
It Just Does Not Work. It can't ever really work, not on its own.
A decent case can be made that it was never meant to work, even.
Borders are nothing more than the formal expression of political and military claims.
They are meaningless in and of themselves, and will and must submit to the changing political climates, which are by their turn determined by demographic realities.
Expecting people to simply be aware of borders and have no need nor motivation to ever cross them... is naive and pointless. And that is before considering how oppressive it would be to actually demand people not to consider moving across borders, intermarrying, interbreeding.
Are we truly expected to pretend that people from the other side of a border are some sort of alien creatures that can't be reasoned with, with whom we should not aim to establish joint projects, not try to understand and cooperate with? Are we expected to want to create such a sorry state of things?
It dismays me something fierce that such lines of thought, which I used to believe to be rightfully dead for good, have arisen from the underworld to shamble their ways among human prey and haunt us once more.
That is not to be.
There are no true "ethnic groups". No true "nations". There are people, there is land, there are cultures. The land is pretty much immutable. The people and the cultures should savor their own dynamism and ever-changing boundaries and thrive on it, without ever lending borders and nationalisms much significance at all.