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Before the Mongol invasion (1235–41)

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 principal 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 contemporaneous Roger of Torre Maggiore, he even "had the chairs of the barons burned" in order to prevent them from sitting in his presence during the meetings of the royal council. Béla set up special commissions which revised all royal charters of land grants made after 1196. The annulment of former donations alienated many of his subjects from the King. Pope Gregory IX protested strongly at the withdrawal of royal grants made to the Cistercians and the military orders. In exchange for Béla's renouncing of the taking back of royal estates in 1239, the Pope authorized him to employ local Jews and Muslims in financial administration, which had for decades been opposed by the Holy See.

After returning from Magna Hungaria in 1236, Friar Julian informed Béla of the Mongols, who had by that time reached the Volga River and were planning to invade Europe. The Mongols invaded Desht-i Qipchaq —the westernmost regions of the Eurasian Steppes —and routed the Cumans. Fleeing the Mongols, at least 40,000 Cumans approached the eastern borders of the Kingdom of Hungary and demanded admission in 1239. Béla only agreed to give them shelter after their leader, Köten, promised to convert together with his people to Christianity, and to fight against the Mongols. However, the settlement of masses of nomadic Cumans in the plains along the Tisza River gave rise to many conflicts between them and the local villagers. Béla, who needed the Cumans' military support, rarely punished them for their robberies, rapes and other misdeeds. His Hungarian subjects thought that he was biased in the Cumans' favor, thus "enmity emerged between the people and the king", according to Roger of Torre Maggiore.

Béla supported the development of towns. For instance, he confirmed the liberties of the citizens of Székesfehérvár and granted privileges to Hungarian and German settlers in Bars (Starý Tekov, Slovakia) in 1237. Zadar, a town in Dalmatia which had been lost to Venice in 1202, acknowledged Béla's suzerainty in 1240.