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Duke of Transylvania (1226–35)

King Andrew transferred Béla from Slavonia to Transylvania in 1226. In Slavonia, he was succeeded by his brother, Coloman. As Duke of Transylvania, Béla adopted an expansionist policy aimed at the territories over the Carpathian Mountains. He supported the Dominicans ' proselytizing activities among the Cumans, who dominated these lands. In 1227 he crossed the mountains and met Boricius, a Cuman chieftain, who had decided to convert to Christianity. At their meeting, Boricius and his subjects were baptized and acknowledged Béla's suzerainty. Within a year, the Roman Catholic Diocese of Cumania was established in their lands.

Béla had long opposed his father's "useless and superfluous perpetual grants", because the distribution of royal estates destroyed the traditional basis of royal authority. He started reclaiming King Andrew's land grants throughout the country in 1228. The Pope supported Béla's efforts, but the King often hindered the execution of his son's orders. Béla also confiscated the estates of two noblemen, brothers Simon and Michael Kacsics, who had plotted against his mother.

Béla's youngest brother, Andrew, Prince of Halych, was expelled from his principality in the spring of 1229. Béla decided to help him to regain his throne, proudly boasting that the town of Halych "would not remain on the face of the earth, for there was no one to deliver it from his hands", according to the Galician–Volhynian Chronicle. He crossed the Carpathian Mountains and laid siege to Halych together with his Cuman allies in 1229 or 1230. However, he could not seize the town and withdrew his troops. The Galician–Volhynian Chronicle writes that many Hungarian soldiers "died of many afflictions" on their way home.

Béla invaded Bulgaria and besieged Vidin in 1228 or 1232, but he could not capture the fortress. Around the same time, he set up a new border province, the Banate of Szörény (Severin, Romania), in the lands between the Carpathians and the Lower Danube. In a token of his suzerainty in the lands east of the Carpathians, Béla adopted the title "King of Cumania" in 1233. Béla sponsored the mission of Friar Julian and three other Dominican friars who decided to visit the descendants of the Hungarians who had centuries earlier remained in Magna Hungaria, the Hungarians' legendary homeland.