Battle of Ballon
845 CE.
Shown on the map at 47.704°, -2.070°.
The Battle of Ballon took place on 22 November 845 between the forces of Charles the Bald, king of West Francia, and Nominoë, Duke of Brittany. Nominoë was appropriating border territory and opposing Charles' attempt to impose Frankish authority. Nominoë defeated Charles, initiating a period of Breton expansion and consolidation of power.
Text from the Wikipedia article Battle of Ballon, licensed CC BY-SA 4.0.