Chivalry and Betrayal: The Hundred Years' War
In 1435 Bedford died and sworn enemies Charles VII and Phillip, Duke of Burgundy joined forces to get the English out and with Henry VI reluctance to wage war and to maintain peace by marrying Margaret of Anjou. Charles had no intention of peace and used cannon and sieges to retake France and Normandy culminating in the defeat of the English at the Battle of Castillon in the last English territory of Gascony ending the hundred years war.
Brittany
The Breton War of Succession, a local episode of the Hundred Years' War, saw the House of Blois, backed by the French, fighting with the House of Montfort, backed by the English. The Montforts won in 1364 and enjoyed a period of total independence until the end of the Hundred Years' War, because France was weakened and stopped sending royal envoys to the Court of Brittany. English diplomatic failures led to the Breton cavalry commanders Arthur, Comte de Richemont (later to become Arthur III, Duke of Brittany) and his nephew Peter II, Duke of Brittany playing key roles on the French side during the deciding stages of the War (the Battle of Patay, the Treaty of Arras (1435), the Battle of Formigny and the Battle of Castillon). However, Brittany lost the Mad War against France in 1488, mostly because of its internal divisions which were exacerbated by the corruption at the court of Francis II, Duke of Brittany. Indeed, some Breton lords were fighting on the French side.