Sir John Chandos and Sir Hugh Calveley were leading figures who joined de Montfort and formed the Anglo-Breton army. Prior to the battle, Charles de Blois and Breton lords who supported de Montfort tried to negotiate a peace but such was the English insistence that battle commence, any hope of reconciliation and division of the duchy was swept aside. When John de Montfort laid siege to Auray, Charles de Blois, well supported by French and Breton lords and mercenaries, hurried to trap de Montfort with his back to the sea, but on the day of the battle many of his Breton mercenaries deserted. When Charles de Blois was killed, and the battle won, it is recorded that English and Breton men-at-arms stuck their pennons and lances in a hedge and stripped off their armour and mail to cool down.

The Prince of Wales and Aquitaine needed safe borders for the Duchy of Aquitaine. He had charmed Gascon lords with his banquets and tourneys and when King Don Pedro I of Castile and León in Spain was threatened by a mercenary invasion led by French, Breton and English mercenaries, he knew the stated aim of waging a crusade against the Islamic Emirate of Granada, allies of Don Pedro lying south of Castile, was a pretext. The French intended to place Don Pedro’s half-brother, the bastard Henry of Trastámara, on the throne. This French puppet would give the recently anointed French King, Charles V, a strategic advantage by placing an enemy at the Prince’s back across the border. The duplicitous King of Navarre eventually played along with the French and closed his border to Gascony, which cut off any chance of Don Pedro escaping and avoiding death at his half-brother’s hand. Don Pedro’s reputation was one of a licentious adulterer, a vicious King who slew hundreds, who embraced friendship with the Muslim South and was accused of having his young French bride, Blanche de Bourbon, murdered.