Æthelfrith, king of Bernicia, gains control of Deira around 604; the circumstances of this are unknown.

That he gained Deira through conquest is suggested by the exile of Edwin, son of the former king Ælla, and Hereric, Edwin's nephew, who are both notable members of the Deiran royal line; the short five-year reign of Aethelric of Deira, who ruled immediately prior to Æthelfrith's acquisition of Deira, may also indicate conquest.

D. P. Kirby on the other hand suggested that Æthelfrith's rule of both kingdoms may have represented "a formalization of an existing relationship" of cooperation between the two.

Kirby also pointed out that Edwin did not necessarily go into exile immediately, and considered it likely that Æthelfrith's hostility towards him "manifested itself only by degrees".

It is also around 604 that Æthelfrith's son Oswald is born.

Oswald's mother is Acha, daughter of Ælla, and thus Edwin's sister.

Although Bede does not explicitly say Æthelfrith married Acha, it is thought that he did so; he may have married her prior to taking power in Deira, in which case the marriage may have facilitated it, or he may have done so afterwards in order to consolidate his position there.

Edwin, installed on Æthelfrith’s vacant throne by Raedwald, becomes king not just of Deira but of Bernicia as well.

His reign marks the domination of Northumbria as leading Anglo-Saxon state of the British Isles.

Edwin of Northumbria invades and annexes the minor British kingdom of Elmet in 616 (or 626).

The Deiran exile Hereric had been poisoned while at the court of Ceretic, king of Elmet; Æthelfrith may have been responsible for this killing.

Edwin, apparently seeking safety from Æthelfrith, seems to have traveled between many different kingdoms during his period of exile.

He may have spent time during his exile in the British kingdom of Gwynedd, and it seems clear that he spent time in Mercia, because he married a daughter of king Cearl.

Edwin ended up in East Anglia, under the protection of its king, Raedwald.

Æthelfrith had sent messengers to bribe Raedwald with "a great sum of money" into killing Edwin; Bede reports that his first message had no effect, but Æthelfrith had sent more messengers and threatened war if Raedwald did not comply (bribes and threats of this kind may have previously been used to accomplish Hereric's killing.)

Raedwald had eventually agreed to kill Edwin or hand him over to Æthelfrith's messengers, but was reportedly dissuaded from this by his wife, who said that such a thing was unworthy of his honor.

Instead, Raedwald raises an army and marches against Æthelfrith, and around 616 Æthelfrith is defeated and killed on the east side of the River Idle by an army under Raedwald; Bede says that Æthelfrith had the inferior army, because Raedwald had not given him time to bring all his forces together.

While presented by Bede as being fought simply over the issue of Edwin, this war may have actually involved questions of power and territory between the two rulers.

Æthelfrith's heir, prince Eanfrith, seeks refuge with his mother's family, probably in Gododdin (modern Scotland) and moves further north into Pictland.

Æthelfrith's younger sons Oswald and Oswiu escape to the court of king Eochaid Buide of Dál Riata, where they are soon converted to Christianity by the monks of Iona.

Æthelfrith’s son Oswald, following the death of his father, moves to the Scottish island of Iona and is there converted to Christianity.

The Angles under Edwin of Northumbria invade Rheged ("Old North") in Northern England and expel king Llywarch Hen.

He flees to Powys and becomes a famous bard.

Edwin's armies fight also against Gododdin and Strathclyde.

King Edwin marries Æthelburga of Kent, who, as a Christian, brings her personal chaplain, Paulinus, and encourages her husband to convert to Christianity.

Paulinus, last of the missionaries sent to England by Pope Gregory I, builds a wooden church in the old Roman legionary headquarters in York and baptizes Edwin of Northumbria as the first Christian king in Northern England, triggering the initial phase of the conversion of the pagan north.

It is possible that Eanfrith had been initially cooperative with Cadwallon.

The historian D. P. Kirby, pointing to Eanfrith's evident ability to quickly exploit Edwin's death, has speculated that "a wide-ranging set of alliances" that included both Cadwallon and Eanfrith may have existed. (Kirby, The Earliest English Kings (1991, 2000), page 73.)

If there was a friendly relationship between Eanfrith and Cadwallon at first, however, it must not have lasted, since Bede reports that Eanfrith went to Cadwallon "with only twelve chosen soldiers" in an attempt to negotiate peace, but Cadwallon had him killed.

King Oswald, owing to his historical connection to Iona's monastic community, had requested that missionaries be sent from that monastery instead of the Roman-sponsored monasteries of Southern England.

They had at first sent him a bishop named Cormán, but he returned in abject failure to Iona and reported that the Northumbrians were too stubborn to be converted.

Aidan of Lindisfarne, an Irish missionary, had criticized Cormán's methods and is soon sent in 635 as his replacement.

Anglo-Saxon paganism in the years prior to Aidan's mission, was argely displacing by Christianity, which had been propagated throughout Britain (but not Ireland) by the Roman Empire.

Bastions of Christian thought continue to thrive, though it seems a foregone conclusion that the region is returning to its indigenous religion.

Aidan establishes a bishopric on the holy island of Lindisfarne and works successfully to reestablish Christianity in Northumbria.