During that conflict, he raised a guerrilla force of archers which opposed the otherwise total occupation of the south-east by Prince Louis of France.
A contemporary chronicler, Roger of Wendover, wrote of him: A certain youth, William by name, a fighter and a loyalist [to King John] who despised those who were not, gathered a vast number of archers in the forests and waste places [of the Kent and Sussex Weald], all of them men of the region, and all the time they attacked and disrupted the enemy, and as a result of their intense resistance many thousands of Frenchmen were slain.
On the way, William's force ambushed Louis near Lewes, routing them and pursuing them to Winchelsea, where they only escaped starvation thanks to the arrival of a French fleet.
At the end of the war, William was granted a pension from the crown and made warden of the Weald and (on 28 May 1241) Sergeant of the Peace (predecessor title to that of Provost Marshal, now head of the Royal Military Police) in reward for his services.
Until his death, he filled this post, collected his pension and fulfilled minor duties such as fetching logs for the royal household.