War of the Breton Succession The War of the Breton Succession was a conflict between the Counts of Blois and the Montforts of Brittany for control of the Duchy of Brittany . It was fought between 1341 and 1364 . It formed an integral part of the early Hundred Years ' War due to the involvement of the French and English governments in the conflict ; the French supported the Blois whilst the English backed Montfort . Although Montfort was ultimately successful following the Battle of Auray in 1364 , it was the French who were to gain the most from his victory . Background . The dukes had both a historical and ancestral connection to England and were also Earls of Richmond in Yorkshire . Duke Arthur II of Dreux married twice , first to Mary of Limoges ( 1260 – 1291 ) , then to Yolande of Dreux , countess of Montfort ( 1263 – 1322 ) and widow of king Alexander III of Scotland . From his first marriage , he had three sons , including his heir John III and Guy , count of Penthièvre ( d . 1331 ) . From Yolande , Arthur had another son , John , who became count of Montfort . ( See Dukes of Brittany family tree . ) John III strongly disliked the children of his father ' s second marriage . He spent the first years of his reign attempting to have this marriage annulled and his half - siblings bastardized . When this failed , he tried to ensure that John of Montfort would never inherit the duchy . Since John III was childless , his heir of choice became Joan of Penthièvre , " la Boiteuse " , daughter of his younger brother Guy . In 1337 she married Charles of Blois , the second son of a powerful French noble house and son of the sister of King Philip VI of France . But in 1340 , John III reconciled himself with his half - brother , and made a will that appointed John of Montfort the heir of Brittany . On 30 April 1341 , John III died . His last words on the succession , uttered on his deathbed , were , " For God ' s sake leave me alone and do not trouble my spirit with such things . " First stage of the war . Most of the nobility supported Charles of Blois , so if John of Montfort was to have any chance , he was dependent upon swift action before organized resistance could be made . John quickly took possession of the ducal capital Nantes and then seized the ducal treasury at Limoges . By the middle of August , John of Montfort was in possession of most of the duchy , including the three principal cities , Nantes , Rennes and Vannes . Up to this point , the succession crisis had been a purely internal affair . But to complicate things further , the Hundred Years ' War between England and France had broken out four years earlier , in 1337 . In 1341 , there was truce between the two countries , but there was little doubt that hostilities would be renewed when the truce ended in June 1342 . Thus , when rumours reached Philip VI of France that John of Montfort had received English agents , the French Crown naturally took a more direct interest in the situation . Charles of Blois became the official French candidate . Whatever had been his original intentions , John of Montfort was now forced to support Edward III of England as King of France . Edward III was bound by the truce not to take any offensive action in France . Nothing in it , however , hindered France from subduing rebellious vassals . In November , after a short siege and defeat at the battle of Champtoceaux , John of Montfort was forced to surrender at Nantes by the citizens . He was offered safe conduct to negotiate a settlement with Charles of Blois , but when this led nowhere he was thrown in prison . It now fell upon John ' s wife , Joanna of Flanders to lead the Montfortist cause . Deeming her possessions in the east undefendable , she set up headquarters at Hennebont in western Brittany but was driven into Brest and besieged , the siege being broken by the arrival of an English army under the Earl of Northampton at the naval battle of Brest . In Paris it was feared that Edward III would land at Calais once the truce ran out . The major part of the French army was therefore withdrawn , and Charles of Blois was left to pursue his claim on his own . Charles soon proved himself to be an able soldier : Rennes and Vannes were taken and many of the Montfortist captains defected . In late November , Edward III arrived with his army at Brest . He almost at once marched against Vannes . The siege dragged on and a French army was assembled to meet him , but 19 January 1343 , before any major engagements could be fought , the two kings agreed upon a new truce . Vannes was taken into papal custody . With John of Montfort in prison , his son an infant , and his wife recently gone mad , the places under Montfortist control were in practice administered from London , with a large permanent English garrison at Brest . The truce was to last until 29 September 1346 with the hopes that in the meantime the disputes between the two kingdoms could be permanently settled , but in Brittany it made little difference . The truce bound the two kings and their followers , but Charles of Blois claimed to be fighting his own separate war , and was therefore not bound by any truce . The brutal small scale fighting continued at the same pace . In Paris , John of Montfort was released from prison 1 September 1343 in return for a huge bond and a promise to stay on his estates in the east . The English coastal garrisons held firm , but the Montfortist party continued to crumble . They had some successes , such as the expulsion of the papal custodians from Vannes , but with no unifying leadership , mostly they were reduced to pleading for men and money from London . To hamper communication between Brest and Vannes , Charles of Blois laid siege to Quimper in early March 1344 . The city fell by assault 1 May and , as usual at that time , this meant the slaughter of civilians in huge numbers , between 1400 and 2000 . The English prisoners were held for ransom , but the Breton and Norman captives were dispatched to Paris where they were executed for treason . During the summer and autumn , the Montfortist party fell apart . Even those who had been John of Montfort ' s staunchest allies now considered it futile to continue the struggle . It therefore mattered little that in March 1345 John finally managed to escape to England . With no adherents of note of his own , he was now little more than a figurehead for English ambitions in Brittany . Edward III decided to repudiate the truce in summer 1345 , a year before it was due to run out . As part of his larger strategy , a force was dispatched to Brittany under the joint leadership of the Earl of Northampton and John of Montfort . Within a week of their landing in June , the English had their first victory when Sir Thomas Dagworth , one of Northampton ' s lieutenants , raided central Brittany and defeated Charles of Blois at Cadoret near Josselin . The follow - up was less impressive . Further operations were delayed until July when Montfort attempted the recapture of Quimper . However , news had reached the French government that Edward ' s main campaign had been cancelled and they were able to send reinforcements from Normandy . With his strengthened army , Charles of Blois broke the siege . Routed , Montfort fled back to Hennebont where he fell ill and died 16 September . The heir to the Montfortist cause was his 5 - year - old son , John . During the winter , Northampton fought a long and hard campaign with the apparent objective of seizing a harbour on the north side of the peninsula . Edward III had probably planned to land here with his main force during summer 1346 . However , the English achieved very little for their efforts . Northern Brittany was Jeanne de Penthievre ’ s home region and resistance here was stiff . In the end , Edward decided upon Normandy as the landing spot for his 1346 campaign . Northampton was recalled and Thomas Dagworth was appointed as deputy lieutenant . It was during a tour through the English strongholds that on 9 June Dagworth and his escort were trapped by Charles of Blois and his army near Saint - Pol - de - Léon . They dug in on a hill top and fought off all attacks until nightfall when Charles was forced to retreat leaving many of his wounded behind . The tide turns against Charles . At this point events outside Brittany started to have an effect on the war . The French suffered a major defeat at the Battle of Crécy in 1346 , and at Calais in 1347 . Without French support Charles of Blois gradually began to lose ground to the English captains . The memory of the massacre at Quimper increased his unpopularity , and Breton traders had an economic interest in strengthening links to England due to Brittany ' s strategic position between the Atlantic and English Channel . At the Battle of La Roche - Derrien ( 1347 ) Charles was taken prisoner as he tried to recapture the town , which had just been taken by the English . He was jailed for five years at the Tower of London . The English controlled Brest , Quimper and Vannes . Under pressure from Pope Innocent VI , the English , French and Bretons negotiated a peace , while both factions maintained an uneasy balance of power within the duchy . It was during this period that the Combat of the Thirty took place , a famous episode in medieval chivalry . Conflicts between the French and English strongholds of Josselin and Ploërmel were resolved in a duel between thirty Montfortist knights led by Robert Bemborough , and thirty supporters of Charles de Blois led by Jean de Beaumanoir . The combat took place midway between the two towns on March 26 , 1351 . By nightfall the Anglo - Breton Montfortists had lost nine dead against six of the pro - French knights ; the surviving Montfortists were forced to surrender . Though renowned at the time , and later highly romanticised , the combat had no effect on the outcome of the war . Edward III signed the Treaty of Westminster on 1 March 1353 accepting Charles of Blois as Duke of Brittany if the latter undertook to pay a ransom of 300 , 000 crowns , and that Brittany signed a treaty of alliance " in perpetuity " with England , this alliance to be sealed by the marriage of the Montfortist claimant John de Montfort ( son of the earlier John de Montfort ) with Edward ' s daughter Margaret . The marriage required the approval of the King of France and a papal dispensation . Charles de la Cerda , the Constable of France negotiated the deal , but Charles II of Navarre , who needed continuing war between England and France to maintain his own power , decided to intervene by assassinating the Constable . He then switched his support to France in exchange for territory . The treaty was negated , but Charles of Blois had been freed , and returned to Brittany as duke . Final phase . The situation remained in stalemate for some time , with Charles of Blois as " de facto " duke , but with significant territory still controlled by the Montfortists . Outside events again began to have an effect on the conflict . A plague struck France and the king himself was captured by the English at the Battle of Poitiers in 1356 . The French state was virtually paralysed . In 1362 , when the younger John de Montfort reached 22 years of age , King Edward permitted him to return to Brittany . His return was conditioned by a covenant , the covenant not to marry without permission , given in pledge of several fortresses . On arrival John attempted to reach agreement with Charles of Blois to make peace and share Brittany , but Charles ' s wife Joan urged him to resist and crush John . The war resumed in 1363 when Charles de Blois , assisted by Bertrand du Guesclin , won some successes , but when Bertrand left to take control of strongholds in Navarre and Normandy , Charles ' s advance halted at the unsuccessful siege of Bécherel . Another opportunity to negotiate an agreement arose , but again Joan blocked negotiations . John de Montfort moved to besiege Auray with renowned English warlord John Chandos . Charles of Blois and Bertrand du Guesclin came to the rescue of the besieged city but were decisively defeated at the Battle of Auray on 29 September 1364 . This battle marked the end of this long conflict : Charles of Blois was killed and Joan of Penthièvre , finding herself a widow , saw her cause collapse . Du Guesclin was captured and ransomed by Charles V for 100 , 000 franc s . Peace treaty . Peace was concluded on 12 April 1365 by the first treaty of Guerande which established John de Montfort as Duke of Brittany . He did not reject completely the claims of the Penthievre family , and established the law of succession in Brittany : Charles V did not oppose the elevation of John , fearing that he might declare homage to Edward of England , his protector and stepfather . In addition , France was clearly depleted in the context of 100 years war . He therefore recognized the Duke , received his oath , and won the friendship by this policy of the Breton nobility .