Why did it take so long for France to conquer Brittany? (Short Animated Documentary)

The small Duchy of Brittany sat on the north-west frontier of France for over six centuries. Despite France being much larger and much more powerful, its king never managed to conquer the Bretons and instead had to wait to marry into its ruling family. So why not? Why didn't France conquer Brittany in such a long period of time. To find out watch this short and simple animated history documentary.