That doesn't have to stop them. Europe's borders are also constantly redrawn, even in the 21th century. Next to the usual independence movements and annexations

That doesn't have to stop them. Europe's borders are also constantly redrawn, even in the 21th century. Next to the usual independence movements and annexations there's also a number of land swaps that happen due to referendums or in order to produce more sanely shaped borders. I guess the key is that the initiative has to come from the individual states. If it was dictated from the federal level it indeed wouldn't work