Historian David Douglas recounts that after the Battle of Arques, in February 1054, "WILLIAM [OF NORMANDY'S] force was at any rate large enough for it to be divided into two contingents operating to the west and east of the Seine. THE DUKE himself with men from middle Normandy faced the invaders who were advancing under the French king [HENRY I] through the Evrecin. On the other side of the river, ROBERT, COUNT OF EU with Hugh of Gournay, WALTER GIFFARD, ROGER OF MORTEMER, and the young WILLIAM OF WARENNE, came out from their own lands to withstand the eastern incursion [of the French force] under Count Odo and Count Rainald... [which] seems to have been unprepared for this levy from eastern Normandy.
Having entered the duchy by way of Neufchatel-en-Bray, [the French force] advanced to the neighborhood of Mortemer, and there gave itself up to unrestrained rape and pillage. Widely scattered and demoralized, it thus offered itself as an easy target....The slaughter was considerable." The battle was of decisive importance...the Norman victory was complete, and THE KING OF FRANCE withdrew. "The battle of Mortemer reflected a major crisis in Norman history, and never again was DUKE WILLIAM to be faced by so formidable a threat to ...his power." David C. Douglas, "William the Conqueror" (Berkeley: Univ of Calif Press, 1964, p. 68) Turton's "Plantagenet Ancestry".