For a long time, nobility was much more important than nationality. It was so important to have a ruler from one of the accepted noble houses that Greece import

For a long time, nobility was much more important than nationality. It was so important to have a ruler from one of the accepted noble houses that Greece imported a King from Bavaria (Otto of Greece). And they were basically all related. So one sprawling family dominated Europe for centuries. Almost inconceivable from today's viewpoint.