King Penda of Mercia, now a competitor to Oswald of Northumbria, defeats and kills the latter on August 5, 642, in the battle of Maserfield.

He commands a united British and Mercian force which includes the Welsh army of kings Cadafael Cadomedd of Gwynedd and Cynddylan of Pengwern.

The cause of the war that led to Maserfield is unknown.

The Mercians after the battle become dominant in the English Midlands.

Deira, in the southern part of Northumbria, chooses a king of its own, Oswine, while Bernicia in the north (which had been dominant, with Oswald, a member of the Bernician royal line, ruling both Bernicia and Deira prior to Maserfield) is ruled by Oswald's brother Oswiu.

The battle thus leads to the internal weakening and fracturing of the Northumbrian kingdom, a situation which will last until after the battle of the Winwaed, despite Oswine's murder in 651 on the orders of Oswiu.