In 655 he was attacked by Penda, and, after an unsuccessful attempt to buy him off, defeated and slew the Mercian king at the battle of the Winwaed.
In 868 the Mercian king appealed to Ã†thelred and Alfred for assistance against the Danes, who were in possession of Nottingham.
His position was assured, at least temporarily, in 617, when he decided to espouse the cause of the Northumbrian prince Edwin, then a fugitive at his court, and defeated zEthelfrith of Northumbria on the banks of the Idle, a tributary of the Trent, in Mercian territory.
In 654 Anna was slain by Penda of Mercia, and was succeeded by his brother 2Ethelhere, who was killed in 655 at the Winwaed, fighting for the Mercian king against Oswio of Northumbria.
East Anglia was subject to the supremacy of the Mercian kings until 825, when its people slew Beornwulf of Mercia, and with their king acknowledged Ecgberht (Egbert) of Wessex as their lord.