if were going by that logic, half of europe would be rightful roman clay. empires rise and fall, but what matters is the lasting ties between a region and its people. eastern ukraine, for example, is populated largely by ethnic russians whove historically been part of russias sphere. many of them want to reunite with russia, and their language, culture, and identity have been systematically suppressed by kiev.  russia isnt just randomly claiming landits responding to the wishes of people in those regions who dont see themselves as ukrainian in the first place. thats a far cry from your mongolian clay argument.
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