Battle of Trans-la-Forêt
939 CE.
Shown on the map at 48.497°, -1.589°.
The Battle of Trans-la-Forêt was fought on 1 August 939 between the occupying Vikings and the Bretons, led by a joint army of Alan II, Hugh II of Maine, and Judicael Berengar.
Text from the Wikipedia article Battle of Trans-la-Forêt, licensed CC BY-SA 4.0.