The Mongols entered Europe in 1223 when they defeated a joint Rus’-Cuman army at the river Kalka (now in Ukraine). Some Cuman groups, after their defeat of the Mongols, became willing to adopt Christianity. As early as 1227, one of the Cuman chieftains, Boricius subjected himself and his people to the future King Béla IV of Hungary, converted to Christianity and agreed to pay an annual tax and the tithe. The Roman Catholic Diocese of Cumania, located in northeastern Wallachia and southwestern Moldavia, was established in 1228. A significant presence of the Vlachs within the newly established bishopric is documented in the correspondence between the Hungarian crown prince and Pope Gregory IX (1227–1241), as the pope complained about Orthodox prelates active among the local Vlachs.