descendants, and were also sons of kings. At the beginning of the ninth century, England was almost wholly under the control of the Anglo-Saxons. The Midland kingdom of Mercia dominated southern England, but its supremacy came to an end in 825 when it was decisively defeated by Ecgberht at the Battle of Ellendun. The two kingdoms became allies, which was important in the resistance to Viking attacks. In 853, King Burgred of Mercia requested West Saxon help to suppress a Welsh rebellion, and Æthelwulf led a West Saxon contingent in a successful joint campaign. In the same year Burgred married Æthelwulf's daughter, Æthelswith. In 825, Ecgberht sent Æthelwulf to invade the Mercian sub-kingdom of Kent, and its underking, 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 might not have intended a permanent union between Wessex and Kent as they both appointed sons as underkings 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 underkings were not allowed to issue their own coinage. Viking raids increased in the early 840s on both sides of the English Channel, and in 843 Æthelwulf was defeated at Carhampton. In 850 Æthelstan defeated a Danish fleet off Sandwich in the first recorded naval battle in English history. In 851 Æthelwulf and his second son Æthelbald defeated the Vikings at the Battle of Aclea and, according to the Anglo-Saxon Chronicle, "there made the greatest slaughter of a heathen raiding-army that we have heard tell of up to this present day, and there took the victory". Æthelwulf died in 858 and was succeeded by his oldest surviving son, Æthelbald, as king of Wessex and by his next oldest son, Æthelberht, as king of Kent. Æthelbald only survived his father by two years and Æthelberht then for