His participation in Guelph politics and the planned invasion of Lombardy with Charles I of Naples, caused a war with Oberto Pelavicino, the chief Ghibelline commander in the region, in 1264.

William resisted him with determination and effectiveness, occupying the fortresses of Acqui Terme, Tortona, and Novi Ligure, and affirming his hold on Nizza Monferrato.

William allied with Alfonso X of Castile, who had declared himself the heir of Manfred of Sicily and therefore of the Emperor Frederick II, as the leaders of an anti-Angevin coalition.

In order to cement the alliance also with the Spanish king, the Marquis (widowed since 1270) married Beatrice,[3] Alfonso's daughter, at Murcia in August 1271.

Left alone and seeing his domains under attack by his enemies and Tortona and Acqui lost, William scrambled to form an alliance with the Ghibelline cities of Pavia, Asti, and Genoa.

Having become the military leader of various Lombard cities, including Pavia, Vercelli, Alessandria, Tortona, Genoa, Turin, Asti, Alba, Novara, Brescia, Cremona, and Lodi, he was also elected head of the anti-Angevin coalition.

His daughter by Beatrice of Castile, Violante (Yolanda), married the Byzantine Emperor Andronicus II Palaeologus, taking the name Irene (Eirene).

Having reduced Alessandria to submission, the citizens of Asti paid the Alessandrians a large sum of money and induced them to revolt against the Marquis again.

Heeding the appeals of the citizens, he entered the city to negotiate a peace, but was imprisoned in a cage, iron or wooden, for a public exhibition, until death released him 18 months later, probably due to hunger.

Dante refers to the misery caused in Monferrato and the Canavese by the war with Alessandria in Canto VII of Purgatory: He who the lowest on the ground among them Sits looking upward, is the Marquis William, For whose sake Alessandria and her war Make Monferrat and Canavese weep William left a son, John, who inherited the marquisate.

William's body was given back to his family and was buried in the Cistercian abbey of Santa Maria di Lucedio, alongside his father.

Chivasso, the centre of the Marquis' power, the veritable capital and seat of the marca Aleramica, was but an unimportant provincial town at the time.