The crusade was part of a longstanding conflict between the Drenthers (or Drents) and the bishopric of Utrecht over the prerogatives of the bishop and the religious practices of the Drenthers. The incident which turned the conflict into a crusade was the killing of Bishop Otto II of Utrecht in the Battle of Ane in 1227. Willibrand received papal authorization for a crusade on the grounds, it appears, that the Drenthers were heretics for defying their bishop. He preached the cross in Frisia between the summer of 1228 and the winter of 1230–31. There were several battles, but the crusade ended inconclusively in September 1232. The conflict simmered on into 1234.