Olaf, promptly launching a bid for the former Norse kingdom of York in autumn 937, Olaf crosses the Irish Sea with his army to join the forces of Constantine and Owen, suggesting that the Battle of Brunanburh probably occurs in early October of this year.
Individually Olaf and Constantine are too weak to oppose Æthelstan, but together they could hope to challenge the dominance of Wessex.
Medieval campaigning is normally conducted in the summer, and Æthelstan can hardly have expected an invasion on such a large scale so late in the year.
He seems to have been slow to react.
The allies plunder the northwest while Æthelstan takes his time gathering a West Saxon and Mercian army.
When he marches north, the Welsh do not join him, and they do not fight on either side.
Constantine and Owen come from the north, possibly engaging in some early skirmishes with forces loyal to Æthelstan as they follow the Roman road across the Lancashire Plains between Carlisle and Manchester, with Olaf's forces joining with him en route.
The two sides meet at the Battle of Brunanburh, resulting in an overwhelming victory for Æthelstan, supported by his young half-brother, the future Edmund I. Olaf escapes back to Dublin with the remnant of his forces, while Constantine loses a son.
The English also suffer heavy losses, including two of Æthelstan's cousins, sons of Edward the Elder's younger brother, Æthelweard.
The medieval records of the battle are too elusive to trace the course of the battle with any surety, but the sources consistently describe it as a massive and bloody engagement even within the context of warfare in the Middle Ages.
The famous poem about the battle in the Anglo-Saxon Chronicle records the deaths of five kings and seven earls among Æthelstan's enemies, along with (or among them) Constantine's son.
Æthelweard's Chronicle notes that the battle was still called "the great war" by people in his day.
The largest list of those killed at the battle comes from the Annals of Clonmacnoise and names several kings and princes.
Æthelstan's defeat of the combined Norse-Celtic force facing him confirms England as a fully unified kingdom.
However, he has been militarily weakened and the battle effectively forces all constituent parts of the British Isles to consolidate in the positions they occupy today.
