[soco]

And who gets to decide which moment in time should be taken as reference for the "true" borders? Because borders in Europe changed so much and so many tomes over history, that pretty much everybody can make territorial claims about everybody else and not be totally wrong. I wish I was exaggerating.

[throwaway599281]

You don't even have to look at old maps that much, when as in this case a great part of the population didn't migrate elsewhere.

Just look at the ethnic/linguistic distributions in the area.

This would not work that well for example, German population in territories lost during WW I, afaik.