For many years he tried to find means to ensure that the children of his stepmother, Yolande of Dreux would not inherit the duchy, including trying to have her marriage to his father annulled.

Once in Paris, John was quickly imprisoned and the French courts declared Joan of Penthièvre and her husband Charles of Blois to be the heirs of Brittany.

Beginning the war with a march on Redon, Charles of Blois arrived in 1342 and besieged the town Hennebont, where Joanna was residing for the time being.

Edward, in dire need to become allies with Brittany, found this opportunity impossible to ignore, sending supplies for Joanna to help relieve the siege.

[2] In the siege of Hennebont, she took up arms and, dressed in armor, conducted the defense of the town, encouraging the people to fight, and urging the women to "cut their skirts and take their safety in their own hands".

When she looked from a tower and saw that the enemy camp was almost unguarded, she led three hundred men on a charge, burned down Charles' supplies and destroyed his tents.

Having secured Brest, she gathered together extra supporters and secretly returned to Hennebont, evading the Blois forces and re-entering the town with her reinforcements.

[4] Joanna sailed to England to seek further reinforcements from King Edward, which he provided, but the English fleet was intercepted on its way to Brittany by Charles of Blois' ally, Louis of Spain.

After being initially welcomed with honor, she was later confined by order of King Edward III and spent the rest of her life under house arrest at Tickhill Castle and elsewhere.

Edward entrusted her to the care of Sir William Frank (until 1346), Thomas Haukeston (1346–57), John Delves (d. 1370) and finally to his widow Isabella and Godfrei Foljambe.

Arthur de la Borderie attributed her confinement to mental illness, but more recent research finds no evidence to support this.

de Giston, assisted by his yeoman, would have risked gravely compromising himself by taking her out of the castle in 1347 and attempting to flee with her if she were mentally ill.[6][7] It is likely that Edward imprisoned her in order to increase his own power in Brittany.

He concluded, In those qualities admired by chivalry she was unquestionably an extraordinary woman: courageous and personally valiant, with a head to plan daring exploits and a heart to conduct her through the thick of the danger; impulsive and generous, a free-handed ruler and an admirer of those deeds of chivalrous daring in others which she was so willing to share in herself ... One cannot read her story without enthusiasm, yet one would like to know more of the woman before bestowing unreserved praise on the countess "who was worth a man in a fight" and "who had the heart of a lion".

[2]Joanna was later celebrated for her fiery exploits in Breton folklore, in particular in a ballad collected in Barzaz Breiz, which relates her attack on the camp at Hennebont.