Joan of Penthièvre reigned as Duchess of Brittany together with her husband, Charles of Blois, between 1341 and 1364. Her ducal claims were contested by the House of Montfort, which prevailed only after an extensive civil war, the War of the Breton Succession. After the war, Joan remained titular Duchess of Brittany to her death. She was Countess of Penthièvre in her own right throughout her life.

Joanna of Flanders was Duchess of Brittany by her marriage to John of Montfort. Much of her life was taken up in defense of the rights of her husband and, later, son to the dukedom, which was challenged by the House of Blois during the War of the Breton Succession. Known for her fiery personality, Joanna led the Montfort's cause after her husband had been captured by King Phillip, and began the fight-back. There, she displayed considerable skill as a military leader. Aside from her astounding skills as a military leader, Joanna functioned as a leader that the people loved.