 William and his brother, Odo II of Champlitte joined the Fourth Crusade in September 1200 at Cîteaux.William was one of the crusader leaders who signed the letter written in April 1203 by Counts Baldwin IX of Flanders, Louis I of Blois and Chartres and Hugh IV of Saint Pol to Pope Innocent III who had excommunicated the whole expedition after the occupation of Zara (now Zadar, Croatia).They begged the pope not to chastise Marques Boniface I of Montferrat, the leader of the crusade who had, in order to preserve the integrity of the expedition, withheld the publication of the papal bull of anathema.