Although said to be the most important battle between the northern and southern divisions of the Anglo-Saxons in early Britain, few details are available. For instance, the two armies met on the banks of a river named the Winwaed, but this river has never been identified. (Most likely it was a tributary of the Humber.)
The roots of the battle lay in Penda's success in dominating England in the 630s and 640s. He had defeated Edwin of Northumbria at Hatfield Chase in 633 and Oswald of Bernicia at the Battle of Maserfield in 642. Mercian warriors regularly raided into Northumbria, as far north as Bamburgh, and Deira became a dependent province.
Oswiu presumably resisted Penda's demands, and Penda, after gathering allies from East Anglia and Wales, marched with a force "thirty legions strong". But in the encounter Penda was soundly defeated, both Penda and an unnamed East Anglian king dying.
After the battle, Oswiu took part of Mercia for himself, leaving the rest to Penda's son Peada.