Bela IV (bā´lə, bē´lə), 1206–70, king of Hungary (1235–70), son and successor of Andrew II. He tried to curtail the power of the magnates and set out to recover the crownlands his father had given to supporters. Confronted by the menace of the Mongol invasion, he sent unheeded appeals to Pope Gregory IX and Holy Roman Emperor Frederick II, but he was crushingly defeated at Mohi on the Sajo River in 1241. Returning after the withdrawal of the invaders, he repopulated the country by inviting foreign colonization. In a battle (1246) with the last Babenberg duke of Austria, the duke was killed but the Austrians were victorious. Bela's long struggle with Ottocar II, king of Bohemia, for Austria and Styria ended (1260) in defeat. His last years were disturbed by the rebellion of his son, later King Stephen V.