Oswiu of Northumbria
Mercia had been in conflict with Northumbria since at least 633, when Penda of Mercia defeated and killed Edwin of Northumbria at the Battle of Hatfield Chase. However, there were diplomatic marriages between the two kingdoms: Æthelred's sister Cyneburh married Alhfrith, a son of Oswiu of Northumbria, and both Æthelred and his brother Peada married daughters of Oswiu. Cyneburh's marriage to Alhfrith took place in the early 650s, and Peada's marriage, to Ealhflæd, followed shortly afterwards; Æthelred's marriage, to Osthryth, is of unknown date but must have occurred before 679, since Bede mentions it in describing the Battle of the Trent, which took place that year.