Kingdom of Thessalonica
Following his father's death in 1207, his conquests in Greece, formed into the Kingdom of Thessalonica, passed into the hands of William's half-brother Demetrius. The succession was opposed by the Lombard nobles of the kingdom, led by the regent Oberto II of Biandrate, who preferred the crown to pass to William. William himself was reluctant to claim the throne, and with the support of Latin Emperor Henry of Flanders, the rebellion of the Lombard barons was overcome, and Demetrius was crowned king.