Casimir succeeded his brother Władysław III (killed at the Battle of Varna in 1444) as King of Poland after a three-year interregnum on 25 June 1447. In 1454, he married Elisabeth of Austria, daughter of King Albert II of Germany and Elizabeth of Luxembourg, a descendant of King Casimir III of Poland. Her distant relative was Frederick III, Holy Roman Emperor. The marriage strengthened the ties between the house of Jagiellon and the sovereigns of Hungary-Bohemia and put Casimir at odds with the emperor through internal Habsburg rivalry.