From The Normans in Europe:
The occasion of Rolf's abdication was seized by the Bretons as a fitting opportunity to free themselves from the Norman domination. When Charles the Simple granted Brittany to Rolf at St.Clair-sur-Epte, he made cession of a territory over which he had no real control. So now, the Bretons, roused by the change of rulers at Rouen, rose under two of their princes, Berenger and Alan, massacred the Northmen in their country and invaded the Norman duchy (930-932). William, however, completely crushed the revolt. Berenger submitted, Alan fled to the court of Athelstan, and, when restored all the intercession of the latter, was forced to accept the terms imposed by the conqueror at the first suppression of the rebellion. The result was an important increase of the Norman territory by the acquisition of the Cotentin and the Channel Islands, and the formal acknowledgment of the Norman supremacy over the rest of Brittany.
His next difficulty was the disaffection in Bayeux, where the Northmen witnessed with strong dislike the gradual conversion of the rest of Normandy to the language, manners, and customs of the Franks. William, encouraged by the bravery of his father's trusted adviser, Bernard the Dane, decided to deal firmly with the rebels and (932-3) pouncing on them suddenly, utterly routed them. After the insurrection he strove to crush out the Norse element, and became more thoroughly French than ever. Hence, perhaps, his adhesion given to Rudolf at this date, and his repudiation of the lovely Espriota, his first wife - whom he had married by Norse rite, that is, without religious ties - for Leutgarda, sister of Herbert of Vermandois, and his neglect of Richard, Espriota's son.
Towards the end of his life he changed his policy. He welcomed a fresh arrival of Danes, and allowed them to settle peaceably in the newly acquired district of Cotentin. His son Richard, suddenly emerging from obscurity, became the darling of his father, was entrusted to William's old tutor, Botho, the Danish-born, and Bernard the Dane, and sent to Bayeux to be instructed in the Northern tongue.
Arnulf of Flanders viewed William's partiality to the new arrivals of Northmen with great apprehension, and formed a coalition against him. In December 942, William was treacherously invited by Aruulf to a negotiation with him on the Somme at Pecquigny, separated from his adherents, and basely murdered on the Flemish side of the river. William, who is generally called Longsword by historians, was thus snatched away in the midst of a changeable, aimless life; and the existence of his race and issue in France was endangered by the long rule of a minor.