679. Battle of the Trent: Ecgfrith of Northumbria fights Æthelred of Mercia
Bede mentions this battle (HE, iv.21) as a demonstration of the effectiveness of Archbishop Theodore. In the course of the battle, Ecgfrith's brother Ælfwine, who was also Æthelred's brother-in-law and "much beloved in both kingdoms", was slain. Bede notes that the way was clear for fiercer hostilities between the two peoples, but that Archbishop Theodore intervened to try to keep the peace, and convinced the Northumbrians to accept a money-payment for Ælfwine's death (wergeld) instead of a full slaking of their vengeance in blood. Bede adds that this was followed by a long period of peace between Ecgfrith and Æthelred and their respective kingdoms.
The Anglo-Saxon Chronicle adds no details to Bede's account, and indeed in one version notes only that Ælfwine was slain in 679, with no reference to a battle.