The Battle of the River Idle was a major victory for Rædwald of East Anglia over Æthelfrith of Northumbria in 616 in what is now Nottinghamshire.
Rædwald assembled an army and marched north, accompanied by his son Rægenhere, to confront Æthelfrith.
They met on the western boundary of the kingdom of Lindsey, on the east bank of the River Idle.
After the death of his son, Rædwald furiously breached his lines, killing Æthelfrith amid a great slaughter of the Northumbrians.
Kirby has argued that the battle was more than a clash between two kings over the treatment of an exiled nobleman but was "part of a protracted struggle to determine the military and political leadership of the Anglian peoples" at that time.