c.613. Battle of Chester: Æthelfrith of Northumbria defeats Selyf of Powys
The battle of Chester is noted by Bede (HE, ii.2) as the fulfilment of a prophecy of Augustine's that the British, if they would not convert the English, would be killed by them instead (see entry on 602). Bede states that Æthelfrith (of Northumbria) raised a great army against Chester and there made a great slaughter of the Britons, including over 2000 monks of Bangor who had come to pray for a British victory.
One manuscript of the Anglo-Saxon Chronicle mentions the battle after a note of the death of Pope Gregory, and dates both events to 605. The Chronicle is here following Bede quite closely, and since Bede records Gregory's death in 605 in the chapter before his account of the battle (HE, ii.1), the date was probably originally nothing to do with the battle.
The Annales Cambriae date the battle instead to 613, and this date (or some point between this date and Æthelfrith's death in 616) is generally accepted. The Annales add that Æthelfrith's opponent was Selyf (Solomon) son of Cynan (Cynan was the king of Powys). It has been suggested from contemporary Welsh verse that the kingdom of Powys at this time extended northwards into Lancashire, which might make more sense of the Northumbrian king's attack (Dumville, p.221, notes both the possibility and the lack of real evidence).
D. Dumville, "The origins of Northumbria: some aspects of the British background", in S. Bassett (ed.), The Origins of Anglo-Saxon Kingdoms (London: 1989), pp.213-22