It is just that, although the Franks were unstoppable in Gaul, they repeatedly ran into obstacles in Brittany, unable to conquer this 'Little Britain' region dominated by the Bretons. By the 10th century, due to continuous invasions by Viking pirates, the lords of Brittany voluntarily sought allegiance from the West Frankish King for protection.
However, even after Brittany was incorporated into the Franks, the Bretons remained restless. When William the Conqueror of Normandy claimed the throne of England, many Breton knights joined his ranks, helping him defeat English King Harold II in the Battle of Hastings and enter England.
William the Conqueror, having obtained the throne, elevated those knights from Little Britain to nobility in Great Britain.
The subsequent Hundred Years' War between England and France was also triggered by Henry II's coveting of the lands of the Duchy of Brittany.
