View of the fortress at Klis from west to east

Klis Fortress (seen from its west point, towards east); Béla captured it from Domald of Sidraga, a rebellious Dalmatian nobleman in 1223

The senior king ceded the lands between the Adriatic Sea and the Dráva River—Croatia, Dalmatia and Slavonia—to Béla in 1220. A letter of 1222 of Pope Honorius III reveals that "some wicked men" had forced Andrew II to share his realms with his heir. Béla initially styled himself as "King Andrew's son and King" in his charters; from 1222 he used the title "by the Grace of God, King, son of the King of Hungary, and Duke of all Slavonia".

Béla separated from his wife in the first half of 1222 upon his father's demand. However, Pope Honorius refused to declare the marriage illegal. Béla accepted the Pope's decision and took refuge in Austria from his father's anger. He returned, together with his wife, only after the prelates had in the first half of 1223 persuaded his father to forgive him. Having returned to his Duchy of Slavonia, Béla launched a campaign against Domald of Sidraga, a rebellious Dalmatian nobleman, and captured Domald's fortress at Klis. Domald's domains were confiscated and distributed among his rivals, the Šubići, who had supported Béla during the siege.

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.[31] The Pope supported Béla's efforts, but the King often hindered the execution of his son's orders.[31][32] Béla also confiscated the estates of two noblemen, brothers Simon and Michael Kacsics, who had plotted against his mother.[31][32]

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"[35] 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.

Before the Mongol invasion (1235–41)

Béla's coronation

Béla is crowned king (from the Illuminated Chronicle)

King Andrew died on 21 September 1235. Béla, who succeeded his father without opposition, was crowned king by Robert, Archbishop of Esztergom in Székesfehérvár on 14 October. He dismissed and punished many of his father's closest advisors. For instance, he had Palatine Denis blinded and Julius Kán imprisoned. The former was accused of having, in King Andrew's life, an adulterous liaison with Queen Beatrix, the King's young widow. Béla ordered her imprisonment, but she managed to escape to the Holy Roman Empire, where she gave birth to a posthumous son, Stephen .Béla and his brother Coloman considered her son a bastard.

Béla declared that his main purpose was "the restitution of royal rights" and "the restoration of the situation which existed in the country" in the reign of his grandfather, Béla III. According to the contemporary Roger of Torre Maggiore, he even "had the chairs of the barons burned"