A coalition of Rus' princes and Cuman tribes suffered a sound defeat by the Mongols in the Battle of the Kalka River in 1223. Shortly thereafter Boricius, a Cuman chieftain, accepted baptism and the supremacy of the king of Hungary. The Roman Catholic Diocese of Cumania was set up in his territories in 1228. A letter of 1234 written by Pope Gregory IX refers to a "certain people within the Cuman bishopric called "Walati"" (Vlachs) who even persuaded Catholic Hungarians and Germans to accept the ecclesiastic authority of Orthodox prelates.