Aethelred was a son of King Penda. At about the same time he succeeded his brother to the throne he married Osthryth, daughter of Oswiu of Northumbria. Aethelred attacked Kent in 676 to get Hlothhere of Kent back in line as a vassal of Mercia. He destroyed much of that kingdom as well as sacked Rochester. In 679 Aethelred continued the struggle his father and brother had with Northumbria. He found Ecgfrith of Northumbria waiting for him on the River Trent which divided north and south Mercia. In the battle, Aelfwine, the younger brother of King Ecgfrith was killed and the Northumbrians were defeated. To prevent a blood feud between the two royal families, Theodore, the Archbishop of Canterbury stepped in to prevent further bloodshed. He secured a truce between Northumbria and Mercia that lasted for 50 years. For the rest of his reign Aethelred was respected as overlord of England even though Bede never listed him among the seven .