William IX was marquess of Montferrat from 1494 until his death.
Background
He was the son of marquis Boniface III and Maria Branković, daughter of prince Stefan III of Serbia. After succeeding his father Boniface at his death (1494), he continued the latter"s pro-French policy and, in 1508, he married to princess Anna d"Alençon, daughter of René d"Alençon.
Career
During the Italian Wars, in 1513, he protected the French retreat from Milan. However, to avoid retaliations from Maximilian Sforza William was forced to pay him 30,000 scudi. The Milanese Duke did not respect the treaty, and his troops invaded Montferrat and sacked numerous cities.
At the same time, informed that his relative Oddone, marquess of Incisa, was trying to obtain the title of marquess of Montferrat, William marched to Incisa and occupied that lordship.
However, the annexation was later declared null by Holy Roman Empire diplomacy, but in the subsequent year William was able to maintain the new lands. He had 3 children with Anna d"Alençon:
Maria (1509–1530),
Margherita (1510–1566), married Federico II Gonzaga, Duke of Mantua
Boniface IV (1512–1530).