In 825, Ecgberht sent Æthelwulf to invade the Mercian sub-kingdom of Kent, and its sub-king, Baldred, was driven out shortly afterwards. By 830, Essex, Surrey and Sussex had also submitted to Ecgberht, and he had appointed Æthelwulf to rule the south-eastern territories as King of Kent. The Vikings ravaged the Isle of Sheppey in 835, and the following year they defeated Ecgberht at Carhampton in Somerset, but in 838 he was victorious over an alliance of Cornishmen and Vikings at the Battle of Hingston Down, reducing Cornwall to the status of a client kingdom. When Æthelwulf succeeded, he appointed his eldest son Æthelstan (who died in the early 850s) as sub-king of Kent. Ecgberht and Æthelwulf may not have intended a permanent union between Wessex and Kent as they both appointed sons as sub-kings and charters in Wessex were attested (witnessed) by West Saxon magnates, while Kentish charters were witnessed by the Kentish elite; both kings kept overall control and the sub-kings were not allowed to issue their own coinage.