Relationship between Bulgaria and Hungary deteriorated in the late 1220s. Shortly after the Mongols inflicted a serious defeat on the united armies of the Rus' princes and Cuman chieftains in the Battle of the Kalka River in 1223, a leader of a western Cuman tribe, Boricius, converted to Catholicism in the presence of Andrew II's heir and co-ruler, Béla IV. Pope Gregory IX stated in a letter that those who had attacked the converted Cumans were also the enemies of the Roman Catholic Church, possibly in reference to a previous attack by Ivan Asen, according to Madgearu. Hungarian troops may have tried to capture Vidin already in 1228, but the dating of the siege is uncertain, and it may have occurred only in 1232.