John, King of England
Henry the Young King fought a short war with his brother Richard in 1183 over the status of England, Normandy and Aquitaine. Henry II moved in support of Richard, and Henry the Young King died from dysentery at the end of the campaign. With his primary heir dead, Henry rearranged the plans for the succession: Richard was to be made King of England, albeit without any actual power until the death of his father; Geoffrey would retain Brittany; and John would now become the Duke of Aquitaine in place of Richard. Richard refused to give up Aquitaine; Henry II was furious and ordered John, with help from Geoffrey, to march south and retake the duchy by force. The two attacked the capital of Poitiers, and Richard responded by attacking Brittany. The war ended in stalemate and a tense family reconciliation in England at the end of 1184.

Q: Who became the Duke of Aquitaine in place of Richard?
John