Unable to defend his kingdom against the assaults of the Vikings, the king of the Franks agreed to grant the Cotentin peninsula and the Avranchin, including Mont Saint-Michel traditionally linked to the city of Avranches, to the Bretons in the Treaty of Compiègne (867). This marked the beginning of a brief period of Breton possession of the Mont. In fact, these lands and Mont Saint-Michel were never really included in the duchy of Brittany and remained independent bishoprics from the newly created Breton archbishopric of Dol. When Rollo confirmed Franco as archbishop of Rouen, these traditional dependences of the Rouen archbishopric were retained in it.