The Battle of Mortemer was a defeat for Henry I of France when he led an army against his vassal, William the Bastard, Duke of Normandy[a] in 1054.

[2] The chronicler William of Jumièges reported that the duke's guardian, his teacher and his steward were all killed by rebels.

William defeated the rebels at the Battle of Val-ès-Dunes in 1047, with the support of Henry I of France.

[4] While Duke William confronted the French king to the west of the Seine River, an allied force of Norman barons led by Robert, Count of Eu, Hugh of Gournay, Walter Giffard, Roger of Mortemer, and the young William de Warenne came out of their own lands to stop the incursion by Count Odo and Count Rainald.

[5] All these events were viewed by the French king from his vantage point on Bassenbourg Hill just across the river.