The conflict was a part of the Breton Civil War, a dynastic dispute between two claimants to the Duchy of Brittany which had broken out the previous year.

Philip VI of France was supporting Charles of Blois, his nephew; while Edward III of England had promised military assistance to Joanna, the wife of the rival claimant, John of Montfort.

[7][8][9] Starting in early June 1341 John seized almost all of the fortified places in Brittany and by mid-August had all but made good his claim to the Duchy.

Philip initially disregarded the situation but, correctly suspecting that John was negotiating with the English, had Charles declared the rightful heir on 7 September.

He was willing to commit considerable military resources to achieve this and despatched an army, commanded by his eldest son, John, Duke of Normandy, to support Charles.

In Paris Philip proposed to John that he repudiate all claim to Brittany and his possessions there in favour of Charles of Blois, receiving an annuity and land in France in exchange.

From there Joanna retained control of most of western Brittany and set up her son as the faction's figurehead and heir to his father's claim to the duchy.

[15][16][17] Joanna despatched her senior counsellor, Amaury of Clisson, to Edward III in England with a large sum in cash to encourage English military intervention.

[19] Strategically Edward had the opportunity to set up a ruler in Brittany at least partially under his control, which would provide access to Breton ports, greatly aiding England's naval war and giving ready entry to France for English armies.

Some of the Spanish and Italian mercenaries pushed forward against orders and engaged a group of defenders formed up in front of the town gates.

The Montfortists also fed additional troops into the fight through the gate and Charles's attempt to facilitate an organised retreat failed; the whole attack was thrown back in disorder with many killed.

[32] More than 2,000 English troops landed at Brest on 18 August, relieving the town and causing Charles to withdraw his army from western Brittany.

The English then marched on Morlaix, a town on the north coast with strong fortifications and a secure harbour 50 kilometres (30 mi) from Brest, and besieged it.

[35][note 2] Edward III landed in Brest with a further 3,000 men on 26 October and besieged Vannes, only to agree the Truce of Malestroit on 19 January 1343.

[40] Joanna travelled to England where she went insane and ceased taking any part in the war; she was held in Tickhill Castle until she died in September 1374.

The next year his widow, Joan of Penthièvre, signed the Treaty of Guérande, recognising John of Montfort's son as Duke of Brittany, which ended the war.