Breton War of Succession in the context of John III, Duke of Brittany

The Duchy of Brittany (Breton: Dugelezh Breizh [dyˈɡɛːlɛs ˈbrɛjs]; French: Duché de Bretagne) was a medieval feudal state that existed between approximately 939 and 1547. Its territory covered the northwestern peninsula of France, bordered by the Bay of Biscay to the west, and the English Channel to the north. It was also less definitively bordered by the river Loire to the south, and Normandy, and other French provinces, to the east. The duchy was established after the expulsion of Viking armies from the region around 939. In the 10th and 11th centuries, it was politically unstable, with the dukes holding only limited power outside their own personal lands. The Duchy of Brittany had mixed relationships with the neighbouring Duchy of Normandy, sometimes allying itself with Normandy, and at other times, such as the Breton–Norman War, entering into open conflict.

Henry II of England invaded Brittany in the mid-12th century and became Count of Nantes in 1158 under a treaty with Duke Conan IV. Henry's son, Geoffrey, became Duke through his marriage to Constance, the hereditary Duchess. The Angevins remained in control until the collapse of their empire in northern France in 1204. The French Crown maintained its influence over the duchy for the rest of the 13th century. Monastic orders supported by the Breton aristocracy spread across the duchy in the 11th and 12th centuries, and in the 13th, the first of the mendicant orders established themselves in Brittany's major towns. A civil war broke out in the 14th century, as rival claimants for the duchy vied for power during the Breton War of Succession, with different factions supported by England and France.