The group of six Anglican kingdoms—Kent, Sussex,…

The group of six Anglican kingdoms—Kent, Sussex, Wessex, East Anglia, Essex, Northumbria, and Mercia—is known to historians as the Heptarchy.

Mercia has 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 have been diplomatic marriages between the two kingdoms: Æthelred's sister Cyneburh had married Alhfrith, a son of Oswiu of Northumbria, and both Æthelred and his brother Peada had married daughters of Oswiu.

Cyneburh's marriage to Alhfrith had taken place in the early 650s, and Peada's marriage, to Ealhflæd, had 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 takes place in this year.

Bede does not mention the cause of the battle, simply saying that it occurred in the ninth year of Ecgfrith's reign.

He is more informative on the outcome.

Ælfwine, the young subking of Deira, was killed; Ælfwine was brother to Osthryth and Ecgfrith, and was well liked in both Mercia and Northumbria since Æthelred's marriage to Osthryth.

His death according to Bede threatened to cause further strife between the two kingdoms, but Theodore, the Archbishop of Canterbury, intervened.

Æthelred takes possession of Lindsey again after the battle; the change in control this time will be lasting, and Lindsey will remain part of Mercia until the Viking invasion of the ninth century remakes the map of England.