The Battle of Hatfield Chase was fought on 12 October 633 It pitted the Northumbrians against an alliance of Gwynedd and Mercia. The Northumbrians were led by Edwin and the Gwynedd-Mercian alliance was led by Cadwallon ap Cadfan and Penda. The site of the battle was a marshy area about 8 miles (13 km) northeast of Doncaster on the south bank of the River Don, though this location has been disputed. It was a decisive victory for Gwynedd and the Mercians: Edwin was killed and his army defeated, leading to the temporary collapse of Northumbria.