The first treaty of Guérande, signed on 12 April 1365, ended the Breton War of Succession.
Joanna de Penthievre and her husband Charles de Blois were supported by the King Charles V of France in the contest against John of Montfort, his son John IV and their English allies.
This treaty confirmed Brittany's neutrality in continuing military conflicts between France and England, it granted Joanna de Penthièvre a pension, and it established that if the House of Montfort failed to produce a male heir to the Dukedom, the senior most male heir of Joanna de Penthièvre would become Duke of Brittany.
It had been the insistence by the House of Montfort on male inheritance to the Ducal crown that was at the center of their dispute with Joanna of Penthièvre.
In 1420, descendants of Joanna of Penthièvre, attempted to regain the Duchy by imprisoning John V. The imprisonment failed and upon his release, the Duke forced the surrender of Penthièvre to the Ducal crown, thereby ending one aspect of the Treaty of 1365 meant to favor Joanna and her descendants.