The tumultuous life and career of Jeanne de Penthièvre, Duchess of Brittany, has been overshadowed by her more renowned namesake, Jeanne de Montfort. Following the Breton civil war, later referred to as the War of the Two Jeannes, the latter would ultimately emerge as the founder of a new dynasty, which would rule the Duchy of Brittany until its final annexation by France two centuries later. Thanks to the meticulous scholarship of Erika Graham-Goering of Ghent University, the so-called other Jeanne is being reclaimed as an equally imposing figure of female agency during the early Hundred Years’ War. Graham-Goering's systematic study of known and previously unknown legal and administrative sources from Jeanne de Penthièvre's twenty-three-year-long reign presents Jeanne as a worthy match for her contemporary rulers, including her own celebrated and later canonized husband and co-regent, Charles de Blois.

In 1341, the succession of Charles and Jeanne as Duke and Duchess of Brittany was approved by her uncle, John III and the French king alike, but would soon be challenged by the recently deceased duke's distant Montfort relatives, whose claims were backed by England. Disaster struck in 1347, when Charles was taken prisoner by the English, after which Jeanne ruled Brittany herself for almost a decade while trying to raise the funds to pay the exorbitant ransom for her husband. The money-stricken duchess would pay a heavy price for her accumulating debts, which were rarely settled. She would again be left to fight for herself when Charles was killed in battle in 1364, leaving her with little real power other than her increasingly meaningless title of Duchess of Brittany. The administrative and legal sources scrutinized in this monograph show Jeanne as a competent administrator of her realms but include only rare and elusive glimpses into her own character. The canonization process of Charles de Blois following his death and her dethronement provide some insight into her personal affairs. Eyewitness accounts underscored the governing skills of the duchess, which sometimes overshadowed those of Charles himself. Her attendance at royal councils grew threefold in Charles's absence, and many among his former staff and servants were originally employed by Jeanne. Her dynastic line and extensive landholding throughout Brittany did indeed give her a stronger claim to the duchy than those of her socially inferior husband, whose main asset was that he was a blood relative of the French king.

The granting of lands and favors to a network of cherished vassals was often signed jointly by the two co-rulers, and occasionally “at her insistence” (“a listance dixelle”), as emphasized in their gift of numerous Breton properties to Antoine Doria, admiral of France and Jeanne's captain of La Roche-Derrien. For almost a quarter of a century, Jeanne would try to balance Brittany's precarious and constantly shifting position between France and England, while witnessing demographic collapse brought about by the bubonic plague and trying to keep her many creditors (including the Avignon papacy) at bay. Jeanne would occasionally accept painful compromises, as in the ransom treaty dictated by England's rulers, who at one point held not only her husband but also the upper military hand in Brittany, following a series of French defeats in the 1350s. Throughout her turbulent reign, Jeanne was able to maintain her position as a leading female powerbroker, tied to other European courts and countries through dynastic marriages and determined to keep her titles and lands long after her own formal dethronement.

Graham-Goering's book sheds new light on Jeanne's later career, when she managed to have some of her outstanding debts covered by the French king. She would soon spectacularly break diplomatic relations with him, when France again threatened Brittany's proud but perilous political autonomy. In their joint opposition to France's deadly embrace, the two Jeannes were eventually forced to bury the hatchet. The formerly competing dynasties of Jeanne de Penthièvre and Jeanne de Montfort would henceforth have to coexist until the inevitable demise of the Breton duchy.