 Gregory excommunicated Frederick II on 29 September <unk> , accusing him of breaking his oath to lead a crusade to the Holy Land ; the emperor had dispatched two fleets to Syria , but a plague forced them to return . His wife Isabella died after giving birth to a son , Conrad , in May 1228 . Frederick continued to consider himself king of Jerusalem , in accordance with the precedent set by John during Isabella 's minority . 
