Breton–Norman war

Brittany, an independent Celtic duchy, had a traditional rivalry with neighboring Normandy.

From a historical perspective, the Bretons had steadily lost lands to the Norman's ancestors, the Seine River Vikings.

[1] 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.

Harold Godwinson, the Earl of Wessex and future King of England, saved them.

[4] Duke Conan II was succeeded by his sister, Hawise, whose marriage to Hoel of Cornwall may have been a political move to consolidate and stabilize the east and west regions of the duchy.

Scene from Bayeux Tapestry (c.1066) showing Breton defenders within the castle of Dinan throwing down spears onto the Norman attackers, whilst some of the latter hold burning torches with which to burn down the wooden fortifications
ET CUNAN CLAVES PORREXIT ("and Conan passed out the keys"), scene from Bayeux Tapestry (c.1066) showing Conan II, Duke of Brittany (d.1066) surrendering the keys (hanging from the tip of a lance) to the castle of Dinan , to the forces of William, Duke of Normandy , following the siege and Battle of Dinan