Arthur I of Brittany was born in 1187 and became the Duke of Brittany in 1196. He was the son of Geoffrey Plantagenet, the Duke of Brittany, and Constance of Brittany. Arthur was a key figure in the power struggles of medieval France, particularly during the reign of King John of England, his uncle. Arthur's claim to the English throne made him a rival to King John, leading to conflict. He was captured in 1203 and disappeared under mysterious circumstances, with many believing he was murdered. His death significantly impacted the political landscape of both Brittany and England.