War of the Breton Succession, 1341–1343

The Breton Civil War was a dynastic dispute between two claimants to the Duchy of Brittany which broke out in May 1341, after the death of Duke John III. A complicating factor was the ongoing Hundred Years' War between France and England. Philip VI of France supported the claim to the duchy of his nephew, Charles of Blois; Edward III of England backed the rival claimant, John of Montfort. John seized most of the fortified places in Brittany in 1341, but a French army sent to support Charles overran eastern Brittany and captured John in November. John's wife, Joanna of Montfort, took command of her husband's field army, set up her two-year-old son, also named John, as the faction's figurehead and heir to his father's claim to the duchy, and appealed for English military intervention. A truce between France and England was in place, but it was due to expire in June 1342.

In late May 1342 Charles led an army against the small but strongly walled port of Hennebont where Joanna was based. Repeated assaults were rebuffed, but the delayed arrival of English reinforcements – caused by lack of shipping and bad weather – forced the Montfortists to retreat to the extreme west of Brittany. The English arrived in August, relieved Brest – where Charles was again besieging Joanna – and inflicted a heavy defeat on Charles at the battle of Morlaix, the first land battle of the Hundred Years' War. Edward III arrived with further English reinforcements, besieged Vannes, and in turn overran most of Brittany. Attempts to reinforce or supply Edward from England failed and a French army greatly outnumbering the Anglo-Montfortist force was raised and advanced to Malestroit, 18 miles (29 km) from the English camp. Philip entered into negotiations with Edward and the Truce of Malestroit, which was supposed to pause hostilities for three-and-a-half years, was agreed on 19 January 1343.

A peace conference was arranged in Avignon where, mediated by Pope Clement VI, a treaty to permanently end the war was to be drafted. The conference did not convene until late 1344, due to English quibbling over the arrangements. The proposals made by each side were unacceptable to the other. Neither displayed any willingness to compromise, and the conference rapidly collapsed. After mutual provocations Edward formally renounced the truce on 15 June 1345 and full-scale war resumed. Despite the truce, the Breton Civil War had been grinding on as a disjointed and inconclusive series of petty sieges and skirmishes. The English and their Montfortist allies held almost the whole of Brittany by 1345. The Breton Civil War continued until 1365, the Hundred Years' War until 1453.