William VI of Montferrat

William VI (c. 1173 – 17 September 1226) was the Marquess of Montferrat from 1203 and pretender to the Kingdom of Thessalonica from 1207.

Boniface I's eldest son, and his only son by his first wife, Helena del Bosco, William stood originally to inherit all his father's possessions. He participated in diverse campaigns with his father, including the Battle of Montiglio, in which the men of Asti were defeated in 1191. Between 1193 and 1199, he appeared in many of his father's public acts. On 12 June 1199, he was put in charge of Acqui Terme with twenty knights to combat the Alessandrini, and, on 27 October, he was present near Saluggia for the signing of a pact with the commune of Vercelli.

Boniface I joined the Fourth Crusade as a Christian leader in 1203. In accordance with promises made to Asti and Alessandria, he officially abdicated the marquisate to William before he left. Immediately, William turned towards Asti, then protected by Milan. The Astigiani had a history of rebellion and were growing in power. In August, with his father beside him, he formed an alliance with Alba and Alessandria, another rebellious commune, against Asti. His allies proved of little worth as he had to make many concessions to them and was still defeated in the field. In April 1206, he opened negotiations with Asti. The peace treaty was embarrassing for Montferrat, but it was accepted by all three allies. William promised to get his father's ratification, but his father was killed fighting against the Bulgarians unaware of any peace back home.

Traditionally, the Aleramici adhered to the Ghibelline faction, which supported the Hohenstaufen and their Italian schemes. William, however, lent his support to Otto of Brunswick, the Guelph claimant to the imperial title. Though William expected to see the power of an emperor levelled against his foes, the only aid he received from Otto was directed against small local potentates which posed little real threat. The only great success of the alliance was the sack of Cuneo.

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