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Ladislaus the Posthumous was a Hapsburg who ruled the Archduchy of Austria and the Kingdoms of of Bohemia and Hungary in personal union. However, he died in 1457 at 17, and the three-way personal union died with him.
So, what if he lives to say, 50 or 60 like most medieval kings, has an heir, and gets said heir to inherit all his realms? Austrian troops at Mohacs? The Austro-Hungarian world power of the 18th and 19th centuries comes into being three centuries early?