Æthelfrith (died c. 616) was a King of Bernicia (c. 593 - c. 616) and later, of all Northumbria (c. 604 - c. 616). He succeeded his father, Aethelric of Bernicia . He was the first king to unite Northumbria, which had previously been divided between the kingdoms of Deira and Bernicia, and he was especially notable for his successes against the Britons.

Around the year 598, it was probably Aethelfrith who defeated the northern British in a major battle at Catraeth ; such successes paved the way for Anglian territorial expansion at the expense of the Britons.

In 603 he repelled the attack of Aidan, king of the Dalriad Scots, at Daegsastan, defeating him with great loss. The appearance of Hering , son of Hussa, Æthelfrith's predecessor, on the side of the invaders seems to indicate family quarrels in the royal house of Bernicia.

Aethelfrith invaded Deira in 604, conquering it and killing its king, Aethelric. He subsequently married Acha, daughter of the former King Aella of Deira (father of Aethelric). A son of Aelle, Edwin, fled to the Welsh kingdom of Gwynedd, and Aethelfrith regarded him as serious threat.

Later in his reign, probably in 613, Aethelfrith attacked the Welsh and defeated them in a great battle at Chester. He also massacred the monks of Bangor who were assembled to aid them by their prayers. This war may have been due partly to Æthelfrith's persecution of Edwin, but it had a strategic importance in the separation of the northern Welsh from the Strathclyde Britons.

Edwin fled to East Anglia, where he was protected by Raedwald. Aethelfrith unsuccessfully attempted to bribe Raedwald into killing Edwin. A war followed, and around the year 616 Æthelfrith was defeated and killed at the river Idle by an army under Raedwald. Raedwald then installed Edwin as king of Northumbria.

Preceded by: Hussa	King of Bernicia	Succeeded by: Edwin
Aethelric	King of Deira

This entry was originally from the 1911 Encyclopedia Britannica.