The Bosnian Crusade was fought against unspecified heretics from 1235 until 1241. It was, essentially, a Hungarian war of conquest against the Banate of Bosnia sanctioned as a crusade. Led by the Hungarian prince Coloman, the crusaders succeeded in conquering only peripheral parts of the country. They were followed by Dominicans, who erected a cathedral and put heretics to death by burning. The crusade came to an abrupt end when Hungary itself was invaded by the Mongols during the Mongol invasion of Europe. The crusaders were forced to withdraw and engage their own invaders, most of them perishing, including Coloman. Later popes called for more crusades against Bosnia, but none ever took place. The failed crusade led to mistrust and hatred for Hungarians among the Bosnian population that lasted for centuries.
Several crusades were called against Bosnia, a country long deemed infested with heresy by both the rest of Catholic Europe and its Eastern Orthodox neighbours. The first crusade was averted in April 1203, when Bosnians under Ban Kulin promised to practice Christianity according to the Roman Catholic rite and recognized the spiritual supremacy of the Pope. Kulin also reaffirmed the secular supremacy of the kings of Hungary over Bosnia. In effect, however, the independence of both the Bosnian Church and Banate of Bosnia continued to grow.[1]
At the height of the Albigensian Crusade against French Cathars in the 1220s, a rumour broke out that a "Cathar antipope", called Nicetas, was residing in Bosnia. It has never been clear whether Nicetas existed, but the neighbouring Hungarians took advantage of the spreading rumour to reclaim suzerainty over Bosnia, which had been growing increasingly independent.[2] Bosnians were accused of being sympathetic to Bogomilism, a Christian sect closely related to Catharism and likewise dualist.[3] In 1221, the concern finally prompted Pope Honorius III to preach a crusade against Bosnia.[2] He repeated this in 1225, but internal problems prevented the Hungarians from answering his call.[1]
Honorius III's successor, Pope Gregory IX, accused the Catholic bishop of Bosnia himself of sheltering heretics, in addition to illiteracy, simony, ignorance of the baptismal formula and failure to celebrate mass and sacraments. He was duly deposed in 1233 and replaced with a German Dominican prelate, John of Wildeshausen, the first non-Bosnian bishop of Bosnia. The same year, Ban Matthew Ninoslav abandoned an unspecified heresy, but this did not satisfy Gregory.[1]
In 1234, Pope Gregory IX issued another call for crusade, and this time Hungary readily responded. While it is possible that the Bosnians had failed to align their church with Rome, the crusade actually served as a perfect excuse for the Hungarians to expand their authority.[1] Gregory promised indulgence to prospective crusaders and entrusted Coloman, younger son of Andrew II and brother of Béla IV, with executing the military action.[3][4] Coloman and his followers were put under protection of the Holy See.[4] Neither the enemies nor the targeted region were precisely named in the letters the Pope sent to Coloman and the Bishop of Bosnia. He referred to "Slavonia", mentioning "lands of Bosnia" only in the letter to the Bishop. It is generally understood that, by "Slavonia", he meant Bosnia and its surroundings, Slavic lands, or even to actual Slavonia. The fact that the Bishop of Bosnia was informed, however, makes it clear that Bosnia itself was targeted. The action seems to have been taken against Bosnians in general as only "heretics" are mentioned; it is implied in one source that the crusade was directed against dualists.[4]