In August 937 Olaf and his army sailed from Dublin[2] to join forces with Constantine and Owen, but they were routed in the battle against Æthelstan.

The poem Battle of Brunanburh in the Anglo-Saxon Chronicle recounts that there were "never yet as many people killed before this with sword's edge ... since the east Angles and Saxons came up over the broad sea".

)[9] Though they had all been enemies in living memory, historian Michael Livingston points out that "they had agreed to set aside whatever political, cultural, historical, and even religious differences they might have had in order to achieve one common purpose: to destroy Æthelstan".

[10] In August 937, Olaf sailed from Dublin[2] with his army to join forces with Constantine and Owen and in Livingston's opinion this suggests that the battle of Brunanburh occurred in early October of that year.

[11] According to Paul Cavill, the invading armies raided Mercia, from which Æthelstan obtained Saxon troops as he travelled north to meet them.

[20] Olaf fled and sailed back to Dublin with the remnants of his army and Constantine escaped to Scotland; Owain's fate is not mentioned.

[20] According to the poem: "Then the Northmen, bloody survivors of darts, disgraced in spirit, departed on Ding's Mere, in nailed boats over deep water, to seek out Dublin, and their [own] land again."

[25] The battle of Brunanburh is mentioned or alluded to in over forty Anglo-Saxon, Irish, Welsh, Scottish, Norman and Norse medieval texts.

The poem relates that Æthelstan and Edmund's army of West Saxons and Mercians fought at Brunanburh against the Vikings under Anlaf (i.e. Olaf Guthfrithson) and the Scots under Constantine.

After a fierce battle lasting all day, five young kings, seven of Anlaf's earls, and countless others were killed in the greatest slaughter since the Anglo-Saxon invasions.

[31] William Ketel's De Miraculis Sancti Joannis Beverlacensis (early 12th century) relates how, in 937, Æthelstan left his army on his way north to fight the Scots at Brunanburh, and went to visit the tomb of Bishop John at Beverley to ask for his prayers in the forthcoming battle.

[32] According to Symeon of Durham's Libellus de exordio (1104–15): John of Worcester's Chronicon ex chronicis (early 12th century) was an influential source for later authors and compilers.

[38] Michael Wood argues that this, together with a similar remark in the Annals of Clonmacnoise, suggests that Anlaf and his allies had established themselves in a centre of Anglo-Scandinavian power prior to the battle.

[13] The mid-12th century text Estoire des Engleis, by the Anglo-Norman chronicler Geoffrey Gaimar, says that Æthelstan defeated the Scots, men of Cumberland, Welsh and Picts at "Bruneswerce".

[39] The Chronica de Mailros (1173–4) repeats Symeon of Durham's information that Anlaf arrived with 615 ships, but adds that he entered the mouth of the river Humber.

In the battle which was fought on this occasion there fell Constantine, king of Scots, and five other kings, twelve earls, and an infinite number of the lower classes, on the side of the barbarians.The Annals of Clonmacnoise (an early medieval Irish chronicle of unknown date that survives only in an English translation from 1627[43]) states that: The Annals of Clonmacnoise records 34,800 Viking and Scottish casualties, including Ceallagh the prince of Scotland (Constantine's son) and nine other named men.

[46] Alex Woolf describes it as a pyrrhic victory for Æthelstan: the campaign against the northern alliance ended in a stalemate, his imperium appears to have declined, and after he died in 939 Olaf acceded to the Kingdom of Northumbria without resistance.

[56] In an article in Notes and Queries in 2022, Michael Deakin questions the philological case for Bromborough as Brunanburh, suggesting that the first element in the name is 'brown' and not 'Bruna'.

[57] Michael Wood (historian), in an article in Notes and Queries in 2017, discusses the alternative spelling Brunnanburh 'the burh at the spring or stream', found in several Anglo-Saxon Chronicle manuscripts.

[62] Many other medieval sources contain variations on the name Brunanburh, such as Brune,[63][64] Brunandune,[65] Et Brunnanwerc,[33] Bruneford,[66] Cad Dybrunawc[67] Duinbrunde[68] and Brounnyngfelde.

The poem in the Anglo-Saxon Chronicle says that the invaders fled over deep water on Dingesmere, perhaps meaning an area of the Irish Sea or an unidentified lake or river.

[41] According to this account, Olaf's army occupied an unnamed fortified town north of a heath, with large inhabited areas nearby.