In 1251 Ottokar 's father King Wenceslaus I of Bohemia had not only granted Giving him the title of a Margrave of Moravia but also installed Placing him as duke of Austria and Styria , territories that were princeless after the ruling Babenberg dynasty Change_of_leadership had become Becoming extinct in 1246 .

To legitimate Legality his succession , Ottokar in 1252 married Forming_relationships the Duchess Margaret of Austria , the sister of the last Babenberg duke and about 26 years his senior .