Breton–Norman war participant Duchy of Brittany
The Breton–Norman War of 1064–1066 was fought between the sovereign Duchy of Brittany and the Duchy of Normandy.
Brittany, an independent Celtic duchy, had a traditional rivalry with neighboring Normandy.Neighboring rivals
From a historical perspective, the Bretons had steadily lost lands to the Norman's ancestors, the Seine River Vikings.
The 1064–1065 animosity between Brittany and Normandy was sparked after William the Conqueror, as Duke of Normandy, supported a Breton, Rivallon I of Dol's rebellion against the hereditary Duke of Brittany, Conan II.In 1065, the year before his invasion of Anglo-Saxon England, William of Normandy sent word to the surrounding countries (including Brittany), warning them against attacking his lands, while he was away on the grounds that his mission bore the papal banner. However, Duke Conan promptly informed the Norman Duke that he would take the opportunity to invade the latter's Duchy.Loss of Breton lands
Duke William's army therefore set out to appease the Breton threat.  While outside the monastery of Mont Saint-Michel, two Norman soldiers became mired in quicksand. Harold Godwinson, the Earl of Wessex and future King of England, saved them.