Arthur I, 1187–1203?, duke of Brittany (1196–1203?), son of Geoffrey, fourth son of Henry II of England and Constance, heiress of Brittany. Arthur, a posthumous child, was proclaimed duke in 1196, and an invasion by his uncle King Richard I of England was repulsed with French aid. Subsequently, Arthur was brought up at the court of King Philip II of France. On Richard's death (1199), Arthur's claim to the English crown was passed over in favor of his uncle John, youngest son of Henry II. Arthur allied himself with Philip II, who invested him with all of Richard's fiefs in France. The nobles of Anjou, Maine, and Touraine recognized Arthur as their ruler, but the young duke was captured (1202) by John while attempting to subdue Poitou. He was imprisoned in Rouen; his fate is uncertain, although John was suspected of murdering him in 1203. His story is told in Shakespeare's King John. Arthur's sister and heir married Pierre Mauclerc, who later became duke of Brittany as Peter I.