Cynewulf was a member of the West Saxon witan. He led that council in expelling Sigeberht. Cynewulf was elected by the witan to rule Wessex in place of Sigeberht. He attested a at the court of King Æthelbald of Mercia. The charter was for Cynewulf to grant land to an abbot in Wiltshire. This indicates Wessex was still a dependency of Mercia. Æthelbald died a few months later. Taking advantage of this Cynewulf took back lands he lost in Berkshire and Wiltshire. Cynewulf was occupied during much of his reign in wars against the Welsh of Dumnonia. He was never able to conqueror Cornwall however. In 779, Cynewulf was defeated by Offa of Mercia at the Battle of Bensington. and Offa then retook Berkshire, and perhaps also London. Despite this defeat, there is no evidence to suggest Cynewulf became subject to Offa (as his successor, Beorhtric was).