Béla's favoritism towards his younger son, Béla (whom he appointed Duke of Slavonia) and daughter, Anna irritated Stephen. The latter suspected that his father was planning to disinherit him. The relationship between father and son remained tense. Stephen seized his mother's and sister's estates which were situated in his realm to the east of the Danube. Béla's army under Anna's command crossed the Danube in the summer of 1264. She occupied Sárospatak and captured Stephen's wife and children. A detachment of the royal army, under the command of Béla's Judge royal Lawrence forced Stephen to retreat as far as the fortress at Feketehalom (Codlea, Romania) in the easternmost corner of Transylvania. The king-junior's partisans relieved the castle and he started a counter-attack in the autumn. In the decisive Battle of Isaszeg, he routed his father's army in March 1265.

It was again the two archbishops who conducted the negotiations between Béla and his son. Their agreement was signed in the Dominican Monastery of the Blessed Virgin on Rabbits' Island (Margaret Island, Budapest) on 23 March 1266. The new treaty confirmed the division of the country along the Danube and regulated many aspects of the co-existence of Béla's regnum and Stephen's regimen, including the collection of taxes and the commoners' right to free movement.