The Duchy of Brittany is a small (6 province) country in the furthest North-western reaches of France, in 1356 it is it divided between two rival houses fighting for it's succession, Monfort, as supported by the King of England, and Blois, as supported by the King of France. At game start, it is this conflict that simulates the ongoing Hundred Years' War.
The Breton war of succession[]
From 1305 to 1312 Arthur II was Duke of Brittany, from his first wife he a son, John and a second son Guy. From his second marraige he had a son called John of Monfort. After his death John became John III of Brittany, despite having three wives he never had children and disliking his father's second marraige attempted to ensure the Monforts would never rise to the ducal throne by bequeathing it to his niece's (by Guy) husband Charles of Blois. John III died in 1341 and John of Monfort immediately siezed the three largest cities of Nantes, Rennes and Vannes attempting to legitimize his succession before organized ressistance by the supporters of Blois can be made.
Meanwhile, the Hundred Years' War.
Philip IV had heard rumours that John had recieved English agents to his court and immediately began to support Charles of Blois as the rightful Duke, and Monfort was forced to ally with Edward III of England to retain his title.