Cotentin Peninsula
King Alan the Great of Brittany (d. 907) waged war successfully on the Norsemen. As the result of his conquests, the Cotentin Peninsula was included theoretically in the territory of the Duchy of Brittany, after the Treaty of Compiègne (867) with the king of the Franks. The Dukes of Brittany suffered continuing Norse invasions and Norman raids, and Brittany lost the Cotentin Peninsula (and Avranchin nearby) after only 70 years of political domination.