Charles I of Naples
In 1266, Charles I of Naples, with the approval of Pope Clement IV, invaded the Kingdom of Sicily, governed by the house of Hohenstaufen, which was the house of Peter's wife, Constance II of Sicily, daughter of Manfred of Sicily and rightful heir to the throne of Sicily after the deaths of her father and cousin Conradin fighting against Charles's invading forces. This made Peter the heir of Manfred of Sicily in right of his wife.