Cynewulf, king of Wessex, had been killed…
789 CE
Cynewulf, king of Wessex, had been killed in 786 by the exiled noble Cyneheard, brother of the former King Sigeberht.
Beorhtric's successful bid for the throne is supported by Offa, king of the Mercians, against Egbert.
It is not entirely clear why Offa intervened in Beorhtric's favor, though it seems likely that the opportunity to influence West Saxon politics, and thus preserve the Mercian Ascendancy, were important factors.
Additionally, it is suggested that Egbert was a descendant of the Kentish dynasty that, under Eahlmund, had rebelled against Offa's rule and beaten him at the battle of Otford.
Beorhtric seems to have been subject to Offa's authority to some extent.
He had held the Synod of Chelsea jointly with Offa in 787, and in 789 he marries one of Offa's daughters, Eadburh.
Land that had traditionally been on the borders of Mercia and Wessex are administered by the Mercian court, as is seen in Charters of Offa, and his son Ecgfrith.
West Saxons seem to have used Offa's currency: a recent survey of early medieval single coin finds reveals a trail of Offa pennies running from the Upper Thames to Wareham, a site connected with Beorhtric.