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.

The House of Montfort-Brittany was a Breton-French noble family, which reigned in the Duchy of Brittany from 1365 to 1514. It was a cadet branch of the House of Dreux; it was thus ultimately part of the Capetian dynasty. It should not be confused with the older House of Montfort which ruled as Counts of Montfort.