According to a non-authentic papal bull written almost a decade later, their territory extended as far as the lower Danube and the "borders of the Brodniks" (the Siret region) in 1222. Papal letters also stated that an unspecified number of Cumans and their wives and children were willing to convert after the Knights defeated them. The Cumans' power diminished sharply after the Mongol victory over a coalition of Rus' princes and Cuman chieftains in the 23 May 1223 Battle of the Kalka River. The Teutonic Knights attempted to overthrow King Andrew, and asked Pope Honorius III to protect their lands. The king invaded the Knights' domain, expelling them in 1225. Andrew made his oldest son, Béla, Duke of Transylvania the following year. Duke Béla, who wanted to expand his authority over the neighboring Cuman tribes, supported the Dominican friars' missionary activities. According to Friar Rudolf of Faenza's testimony during Saint Dominic's canonization, the founder of the Dominican Order "wanted to save all men, Christian and Saracens, but especially the Cumans and other pagans" and expressed "his desire to go to the Cumans and other infidels". The Dominican province of Hungary, one of the order's first territories, was created in the early 1220s. Paulus Hungarus, its first head, "decided to send some virtuous brothers" to the Cumans in the early 1220s; according to The Lives of the Brethren, written during the 1250s by Friar Gerard de Frachet, they were unsuccessful and returned. De Frachet wrote that the next Dominican mission to the Cumans reached the Dnieper River, but the friars "suffered hunger, thirst, lack of covering and persecutions; some of them were held captive and two were killed". Historian Claudia F. Dobre wrote that the "way for the Cumans' conversion was opened" after their defeat at the Kalka River, due to Duke Béla's support of the Dominican missionaries. The nearly-contemporaneous Alberic of Trois-Fontaines wrote that a Cuman chieftain's son visited Robert, Archbishop of Esztergom in Hungary in 1227, asking the archbishop to baptize him and his 12 retainers. The Cuman nobleman also reported that his father and his 2,000 subjects were also willing to come to Transylvania to be baptized. Robert accepted the offer and went to Transylvania with three Hungarian prelates: Bartholomew le Gros, Bishop of Pécs; Bartholomew, Bishop of Veszprém, and Raynald of Belleville, Bishop of Transylvania. According to the chronicle of Emo of Friesland, they met the Cuman