John I, Duke of Brittany was born in 1239 and became the Duke of Brittany in 1237 when he was just a child. He was the son of Peter I, Duke of Brittany and Alix of Thouars. His reign was marked by the ongoing conflict between the French crown and the English crown, as Brittany was a significant territory during the medieval period. John I's rule was short-lived; he died in 1286 at the age of 47. His death led to a succession crisis, as he had no surviving children. This situation ultimately contributed to the ongoing struggles for power in Brittany and its relationship with France and England.