On August 1, 939, a united Breton army led by Alan II, a Breton count Judicael Berengar of Rennes and elements from a Frankish count Hugh II of Maine decisively attacked and defeated the Viking stronghold, bringing an end to the occupation.

The site of the battlefield is considered to be south of Mont St Michel over the river Couesnon within a kilometre from Trans-la-Fôret.

[6] The Breton victory at Trans-la-Forêt freed Brittany of Viking occupation and led to the re-establishment of the Breton State, not as a fully independent Kingdom but as a Sovereign Duchy under Alan II, Duke of Brittany, due to a new fealty owed to the Franks for helping remove the Vikings.

[7] The occupation resulted in Brittany becoming more fortified on the seaward side by returning Breton lords.

The Breton capital was moved from Nantes to inland Rennes as it was regarded as more defensible.