The position of John of Montfort was legally founded on the belief that a brother (even a half-brother) was a closer heir than a beneficiary niece, and that the Salic form of inheritance adopted by the Kingdom of France should be followed.

John returned to Nantes and was recognised as Duke in May 1341 by an assembly composed of members of the towns and minor nobles, but shunned by the major vassals with the exception of Hervé VII, Count of Léon.

Thanks to the ducal treasury, he recruited mercenaries which allowed him to perform in June and July a 'great ride in Brittany' (as described by Arthur de la Borderie) and take control of the duchy.

Having failed to take the domains of Rohan, he submitted Quimper and Brest before descending to Carhaix, and then went back northward and captured Saint-Brieuc, Lamballe, Jugon, Dinan, Dol-de-Bretagne, and returned to Nantes via Ploërmel.

Always accompanied by Hervé VII of Léon, John obtained the submission, if not the support, of some of the major nobles who possessed several of these strongholds and had been loyal to Charles of Blois.

However it would probably require getting Josselin, or the support of the high secular clergy and great lords, or more, to control the domains of the Penthièvres and the House of Rohan, which represented about two-thirds of Brittany.

[9] Charles of Blois then made an appeal to Philip VI, who summoned John of Montfort to a Court of Peers session in Conflans in September 1341.

Philip VI agreed to receive l'hommage lige of Charles of Blois on behalf of his wife, and confiscated from John of Montfort the French lands of the County of Montfort-l'Amaury, as well as the Viscounty of Limoges that he held more unduly.

Charles de Blois failed to take Hennebont, which was defended by Joanna of Flanders, while Robert III of Artois was mortally wounded besieging Vannes in vain leading an English contingent.