Brittany was a duchy in the west of France. Both France and England competed for influence in the duchy; a conflict had to come sooner or later. In 1341, the Breton War of Succession began: France supported Charles of Blois, England John of Montfort - with the result that both were drawn into the war.
In the Sea Battle of Blankenberge 1344, English and Flemish defeated the French; England now ruled the Channel and could land troops in Brittany. In 1346, Brittany saw the Land Battle of Vannes. Thanks to their longbows and the bad weather, the English defeated the French.
1353, France had to make peace with England after the defeat in the battle of Caen. John of Montfort became duke of Brittany, England got all of Gascony (similar as in the peace of OTL 1360), Flanders acquired the Artois.
Despite of the war, during the years 1350-75 - due to the growing demand for food - Brittany and other nations in western Europe extended fishery in the North Atlantic, going farther and farther west. Soon, tales of strange islands in the far west started to spread among fishermen and sailors.
The luck the English had had in the First Aquitainian War turned again: 1439, Brittany got a pro-French ruler, eventually returning to the fold.
In 1469, Brittany sent a ship to the new world; they found the coast of OTL Carolina, but the stronger Dutch didn't allow them to make landfall.
When the French kings strengthened their rule, Brittany lost more and more of its old rights, until it was finally completely annexed.