In 651, King Oswiu had Oswine of Deira killed and replaced by Aethelwald, but Aethelwald did not prove to be a loyal sub-king, allying with the Mercian king Penda; according to Bede, Aethelwald acted as Penda's guide during the latter's invasion of Northumbria but withdrew his forces when the Mercians met the Northumbrians at the Battle of Winwaed.