Following the ratification of the Treaty of Le Goulet, Arthur, offended by Philip, sought refuge with his uncle John. Initially treated kindly, Arthur later grew suspicious and returned to Angers. An undisclosed source mentioned that in April 1202, Arthur was once again engaged, this time to Marie of France, the daughter of Philip II and Matilda of Saxony.
Eleanor of Brittany was engaged with her cousin Henry after the betrayal of Philip II in favor of King John.
Upon his return to France, supported by Philip II, Arthur initiated a campaign in Normandy against John in 1202. Poitou rose in support of Arthur, leading to a revolt. The Duke of Brittany besieged Eleanor of Aquitaine, John's mother, in the Château de Mirebeau. On July 31, 1202, John surprised Arthur by marching on Mirebeau. Subsequently, on August 1, 1202, Arthur was captured by John's barons and confined in the Château de Falaise in Falaise, Normandy.
Hubert de Burgh, under whose guard Arthur resided at the Chateau de Falaise, faced a harrowing decision. According to the contemporary chronicler Ralph of Coggeshall, John issued a command for two servants to blind and castrate the duke. However, de Burgh, unable to allow such mutilation, chose an alternative course of action. Fearing John's wrath, de Burgh discreetly spread word that Arthur had succumbed to natural causes. This announcement incited anger in Brittany, fueling suspicions of Arthur's potential murder.
In the subsequent year, Arthur was transferred to Rouen, entrusted to the care of William de Braose. His disappearance in April 1203 coincided with a backdrop of several triumphant military successes by Philip II of France against King John.
Eleanor of Brittany would establish her rule in Brittany and would wait before she would marry Henry of England and she is convinced that her brother’s foolishness is what killed her brother, she would wait for the situation with her stepfather to be stable before she would marry to Henry of England.
Meanwhile in France, Philip II would petition the Pope to annul the marriage of Joan of England with Prince Louis in 1203 but this would not happen as King John would message the Pope about the intentions of Philip II to marry Eleanor of Brittany to his son after the annulment which would cause the annulment to not happen and Eleanor of Brittany would marry Prince Henry of England in the end of 1203, another supporter of Joan of England is Matilda of Saxony who petitioned Philip II about her, due to the consistent support of Matilda of Saxony to Joan of England, she would name her first child, a daughter born in 1206 as Matilde of France.
They knew that a marriage between Eleanor of Brittany and Louis of France would wipe out the Plantagenet rule in Normandy and Anjou but as Eleanor of Aquitaine is in John’s side he would inherit Aquitaine but he and his mother would choose Richard, his second son as the heir to Aquitaine but that would wait until her death in 1204 and after that, Richard and his wife, Isabel would go to rule Aquitaine.