In 1239, Amadeus was successful in leading the pro-Genevan party over the pro-Savoyard in the episcopal election which placed Jean de Cossonay on the bishop's seat.
[3] In 1236–37, Aymon, Baron of Faucigny, made himself protector of the priory of Chamonix despite the fact that the count of Geneva had a prior right.
The ensuing war, in which Aymon's son-in-law, le Petit Charlemagne, was captured by William's son Rudolf, resulted in Genevan defeat, for on 13 May 1237 William II was forced to accept the judgement of Peter's brother, Amadeus IV, Count of Savoy, condemning him to an indemnity of 20,000 marks and the cession of the strategic Castle Arlod on the Rhône near its confluence with the Valserine, which commanded the approach to the Lyonnais.
[4] In 1250 William II went to war again against le Petit Charlemagne in an effort to check his expansion in the pays de Vaud.
He was also forced to accept the arbitration of Peter's brother, Philip, Archbishop of Lyon, who in turn imposed a "Carthagenian settlement" on 28 June: the count was forced to cede to Peter a large gagerie as security against the payment of an indemnity set at 10,000 marks, which was half of the indemnity ordered in 1237.