Marquis of Montferrat (1207-1225) and claimant to the kingdom of Thessalonica.
Son of Boniface I, marquis of Montferrat, and his first wife Helena del Bosco, William was left to administer his father’s lands after Boniface left Lombardy as one of the leaders of the Fourth Crusade (1202-1204); Boniface subsequently founded a principality in Thessaly and central Greece, after the crusader capture of Constantinople (mod. Istanbul, Turkey) in 1204. William was thus well experienced in government and warfare by the time of his father’s death (1207), and in subsequent years many of Boniface’s Lombard followers were keen to see William installed as king of Thessalonica (mod. Thessaloniki, Greece) in the place of the young and inexperienced Demetrius, Boniface’s son by his second wife, Margaret of Hungary.
By 1222 the Greek successor state of Epiros had conquered large parts of Demetrius’s kingdom, cutting off Thessalonica from Constantinople, and Demetrius himself fled to the West to seek help. Pope Honorius III was now prepared to sanction a crusade under William’s leadership for the defense of the Latin Church in Thessalonica. The crusade was proclaimed on 13 May 1223 and preached in Italy and southern France. Participants were promised an indulgence, and their goods were placed under the protection of the papacy, while a ban was put on the export of horses, weapons, and food to the lands of Theodore Doukas, despot of Epiros. William pledged his own lands to the Emperor Frederick II for 9,000 marks, while further finance was provided by the papacy. The plan for the crusade as it assembled in spring 1224 was for a naval expedition, led by William, Demetrius, and the papal legate, Bishop Nicholas of Reggio, to be coordinated with a land attack from Constantinople by Robert, the Latin emperor. However, William suffered a prolonged illness, and he was not able to sail from Brindisi until 1225. By the time the army arrived in Thessaly, the Frankish garrison of Thessalonica had surrendered to Epirote forces. A dysentery epidemic claimed the lives of many crusaders, including William, and the army broke up. Demetrius returned to Italy and sought refuge at the court of Frederick II. William was succeeded by his son Boniface II (1225-1253), who was able to restore the fortunes of the marquisate.