Mont-Saint-Michel
The king of the Franks could not defend his kingdom against the attacks of the Vikings. The king agreed to give the Cotentin peninsula and the Avranchin, including Mont-Saint-Michel, to the Bretons in the 867 Treaty of Compiègne. For a short time, the mount belonged to the Bretons. In effect, these lands and Mont-Saint-Michel never belonged to the duchy of Brittany. They remained separate bishoprics from the newly created Breton archbishopric of Dol. When Rollo named Franco as archbishop of Rouen, the diocese of Rouen took the lands and the mount. They became part of Normandy once again, but not officially.