creation of a buffer zone along the southern borders. He invaded Bosnia already in the year of his appointment and forced Tzar Michael Asen I of Bulgaria to cede Belgrade and Barancs (Braničevo, Serbia) in 1255. Béla adopted the title of King of Bulgaria, but he only used it occasionally in the subsequent years. The Styrian noblemen rose up in rebellion against Béla's governor Stephen Gutkeled and routed him in early 1258. Béla invaded Styria, restored his suzerainty and appointed his oldest son, Stephen, Duke of Styria. In 1259, Batu Khan's successor, Berke, proposed an alliance by offering to marry one of his daughters to a son of Béla, but he refused the Khan's offer. Discontented with the rule of Béla's son, the Styrian lords sought assistance from Ottokar of Bohemia. Béla and his allies—Daniil Romanovich, Boleslaw the Chaste, and Leszek the Black of Sieradz—invaded Moravia, but Ottokar vanquished them in the Battle of Kressenbrunn on 12 June 1260. The defeat forced Béla to renounce Styria in favor of the King of Bohemia in the Peace of Vienna, which was signed on 31 March 1261. On the other hand, Ottokar divorced his elderly wife, Margarete of Austria, and married Béla's granddaughter—the daughter of Rostislav Mikhailovich by Anna—Kunigunda. Béla had originally planned to give his youngest daughter, Margaret, in marriage to King Ottokar. However, Margaret, who had been living in the Monastery of the Blessed Virgin on Rabbits' Island, refused to yield. With the assistance of her Dominican confessor, she took her final religious vows which prevented her marriage. Infuriated by this act, the King, who had up to that time supported the Dominicans, favored the Franciscans in the subsequent years. He even became a Franciscan tertiary, according to the Greater Legend of his saintly sister, Elisabeth. Civil war (1261–66) Béla and his son, Stephen jointly invaded Bulgaria in 1261. They forced Tzar Constantine Tikh of Bulgaria to abandon the region of Vidin. Béla returned to Hungary before the end of the campaign, which was continued by his son. 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. Stephen often mentioned in his charters that he had "suffered severe persecution" by his "parents without deserving it" when referring to the roots of his conflict with his father. Although some clashes took