King Alan the Great of Brittany waged war successfully on the Norsemen. As the result of his conquests, the Cotentin Peninsula was included in the territory of the Duchy of Brittany. After his death, the Norsemen invaded and occupied Brittany from 907 to 939. While they were eventually expelled by Alan's grandson, Alan II, Duke of Brittany, the subsequent rulers of Brittany were weaker than Alan the Great. The succeeding Dukes of Brittany suffered continuing Norse invasions and Norman raids. Eventually the Cotentin Peninsula was lost to the Brittany, and became part of Normandy.