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. 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. The French Crown maintained its influence over the Duchy for the rest of the 13th century. The Angevins remained in control until the collapse of their empire in northern France in 1204. Henry's son, Geoffrey, became Duke through his marriage to Constance, the hereditary Duchess. 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. The Duchy 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.

The Duchy, in the 10th and 11th centuries, was politically unstable, with the dukes holding only limited power outside their own personal lands. The Duchy was established after the expulsion of Viking armies from the region around 939. It was also less definitively bordered by the river Loire to the south, and Normandy, and other French provinces, to the east.

Its territory covered the northwestern peninsula of Europe, bordered by the Atlantic Ocean to the west, and the English Channel to the north. The Duchy of Brittany ( Breton: Dugelezh Breizh, French: Duché de Bretagne) was a medieval feudal state that existed between approximately 9.

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