The Brittany Civil War, also known as the War of the Breton Succession, occurred in the late 14th century, primarily between 1341 and 1364. It was a conflict over the succession of the Duchy of Brittany, sparked by the death of Duke John III without a clear heir. Two main factions emerged: one supported Charles of Blois, the other backed Jean de Montfort. The war involved various battles and alliances, drawing in external powers like England and France. Ultimately, the conflict ended with the Treaty of Guérande in 1365, which recognized Jean IV as the Duke of Brittany, solidifying the Montfort line.