The War of the Breton Succession (French: guerre de Succession de Bretagne; Breton: Brezel hêrezh dugelezh Breizh) or Breton Civil War was a conflict between the Counts of Blois and the Montforts of Brittany for control of the Duchy of Brittany, then a fief of the Kingdom of France. It was fought between 1341 and 12 April 1365. It is also known as the War of the Two Joans (French: guerre des deux Jeannes) due to the involvement of two rival duchesses of that name, Joan of Flanders and Joan of Penthièvre.
The war formed an integral part of the early Hundred Years' War due to the proxy involvement of the French and English monarchs in the conflict; the French supported the Blois (female heir) whilst the English backed the Montforts (male heir). The rival kings supported the duke of the principle opposite to their own claims to the French throne—the Plantagenet having claimed it by female succession, and the Valois by male succession. Montfort was ultimately successful following the Battle of Auray in 1364 but, in a surprising turn of events, pledged his loyalty to the king of France and not to the Plantagenet king of England who had supported him.