While Richard was away on crusade, Constance took more independence for Brittany, and in 1194 had the young Arthur proclaimed as its Duke.
When Richard died in 1199, his brother John immediately claimed England, but much of the French nobility refused to recognize him as king, preferring Arthur, who declared himself vassal of Philip Augustus. This was sufficient excuse for John, who invaded France in 1202.
Philip having recognized Arthur's right to Brittany, Anjou, Maine, and Poitou, Arthur laid siege to the last-named, but he was surprised at Mirabeau (where he was holding Eleanor of Aquitaine hostage), captured, and imprisoned at Falaise, guarded by Hubert de Burgh. The following year he was transferred to Rouen, under the charge of William de Braose, and then vanished mysteriously in April 1203.
The puzzle of his disappearance gave rise to various stories. One account was that Arthur's jailors feared to harm him, and so he was murdered by John directly and his body dumped in the Seine. William de Braose did rise high in John's favor after Arthur's disappearance, so much so that he was suspected of complicity, and indeed many years later, after difficulties with John, William's wife Maud de Braose directly accused the king of murdering Arthur, which resulted in Maud and her eldest son being imprisoned and starved to death therein. William escaped to France, where he was supposed to have published a statement on what happened to Arthur, but no copy has been found.