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 , to the Bretons in the 867 Treaty of Compiègne .
The king of the Franks was unable to 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 .