Arthur I, Duke of Brittany

Arthur I was born on 29 March 1187 in Nantes, Brittany, as the posthumous son of Duke Geoffrey II and Duchess Constance of Brittany. He was the grandson of King Henry II of England and Eleanor of Aquitaine, and the nephew of King Richard I of England. In 1190, he was named heir presumptive to Richard I, but Richard later designated his brother John as his successor. Arthur was proclaimed Duke of Brittany in 1196, co-ruling with his mother Constance.

Arthur's early life involved political maneuvering, with his mother seeking greater independence for Brittany. In 1199, following Richard I's death, Arthur was supported by Philip II of France in his claim to the English throne. However, the Treaty of Le Goulet in 1200 saw Philip recognize John as King of England, temporarily abandoning Arthur's claim. Arthur continued to assert his rights, leading a campaign in Normandy against John in 1202.

In 1202, Arthur besieged his grandmother Eleanor of Aquitaine at Mirebeau, but was captured by John's forces. He was imprisoned in the Château de Falaise and later transferred to Rouen. His fate became uncertain after his imprisonment, with rumors suggesting he was murdered by John or his associates. His disappearance in 1203 remains a mystery, with no definitive account of his death.

Arthur I disappeared in 1203 at the age of 15 or 16, with his fate unknown and no burial site recorded. He was succeeded as Duke of Brittany by his half-sister, Alix of Thouars.