In 1210, William assisted at the coronation of Jean de Brienne as King of Jerusalem.

He sent Templars to fight in the Battle of Las Navas de Tolosa and in the 1217 Siege of Alcázar, leading to the order flourishing in Spain, achieving its zenith of influence in the area.

The siege lasted eighteen months, with multiple failed assaults on the city and in fighting among crusader leadership spelling disaster for their goals.

Muslim reinforcementsmade their way to Damiette, but William led the Templars in successfully repelling them.

Although the Crusaders were eventually successful in taking the city, William died in 1218 of pestilence, (possibly endemic typhus), as a consequence of being wounded.