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.

Throughout the early 900's, the states of Brittany and West Francia were in a state of flux having both being invaded by the Vikings:

Through the course of three years, the campaign against the Vikings reached a conclusion at Trans-la-Forêt.

On 1 August 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-Forêt.

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.

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

Viking piracy continued to affect a previously vibrant maritime trade between the Bretons and their Welsh cousins, isolating the Bretons.