TheBreton–Norman war of 1064–1066 was fought between the sovereignDuchy of Brittany and theDuchy of Normandy.

Breton–Norman war
Part of theMiddle Ages
"Here the knights of Duke William fight against the men of Dinan; and Conan passed out the keys". Two successive scenes from the contemporaryBayeux Tapestry (c.1066) depicting the Battle of Dinan, one of the decisive battles of the war.
Date 1064–1066 Location Château de Dinan,Brittany Result Norman victory, possible stalemate
Belligerents
Duchy of Normandy Kingdom of England	Duchy of Brittany
Commanders and leaders
Harold Godwinson William, Duke of Normandy Harold Godwinson Rivallon I of Dol-Combourg	Conan II of Brittany

Brittany, an independentCeltic 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.

The 1064–1065 animosity between Brittany and Normandy was sparked afterWilliam the Conqueror, as Duke of Normandy, supported a Breton,Rivallon I of Dol's rebellion against the hereditaryDuke of Brittany,Conan II.^[1]

In 1065, the year before his invasion ofAnglo-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.^[2] However, Duke Conan promptly informed the Norman Duke that he would take the opportunity to invade the latter'sDuchy.

Duke William's army therefore set out to appease the Breton threat. While outside the monastery ofMont Saint-Michel, two Norman soldiers became mired in quicksand.Harold Godwinson, theEarl of Wessex and futureKing of England, saved them.

The Battle of Dinan occurred in 1065. Harold fought on the side of Duke William, whose army had chased Duke Conan fromDol-de-Bretagne toRennes. Duke Conan finally surrendered atChâteau de Dinan,Brittany. The battle is recalled in theBayeux Tapestry (see illustration).^[3]

During Duke Conan's 1066 campaign against Anjou, he tookPouancé andSegré, and arrived inChâteau-Gontier. There he was found dead on 11 December after donning poisoned riding gloves. Duke William was widely suspected.^[4]

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

Bretons would eventually invade England with the Normans in 1066 mainly as cavalry, which they specialized in.