Ever since he had become king of the Wessex and Mercia in 925, Aethelstan had steadily extended his authority. After his grandfather, Alfred the Great, had halted the Norse conquest in 917 Aethelstan's father, Edward the Elder, had taken the East Midlands and East Anglia from the invaders. To consolidate these gains, Edward met Ragnall of York, Constantine King of Alba and the rulers of Bamburgh to establish their spheres of interest. Building on these foundations, in 926 Aethelstan contracted a marriage alliance between the new Hiberno-Scandinavian King of York, Sitric, and his sister. When Sitric died a year later, Aethelstan took advantage and annexed the whole Kingdom of Northumbria including York. This extended his rule and significantly altered the balance of power in the north. A challenge therefore came from the rulers of Alba, Bamburgh and Owain of Strathclyde, who were all threatened by this growing Anglo-Saxon power. However, a show of military force by an emboldened Aethelstan was swiftly followed by a treaty agreed at Eamont in Cumbria where the three northern rulers and Hywel, King of the West Welsh pledged oaths to accept Aethelstan’s authority. Despite this agreement, in 934 Aethelstan invaded the north British kingdoms – possibly because Constantine had broken the peace treaty. Aethelstan ravaged up to the great fortress of Dunnottar, south of Aberdeen and sent his fleet as far as the Viking territories of Caithness, forcing Constantine’s surrender and his and Owain’s acceptance of Aethelstan’s overlordship.

Realising that their only hope of countering the powerful English king was to set aside their differences, former enemies in the north forged a new coalition against him with Constantine and Owain joining forces with Anlaf, the Viking king of Dublin. After defeating the king of Limerick in August 937, Anlaf’s fleet set out for England to reclaim York. It is not recorded how or where the Scots and Strathclyde Welsh met up with the Norse Gaels under Anlaf.

There is no consensus amongst historians of the battle about where Anlaf’s fleet landed, which would begin to indicate where the battle might have been fought. John of Worcester and Symeon of Durham (both early 12^th C) say that Anlaf entered the mouth of the Humber with a strong fleet (615 ships according to Symeon) and at least six other chroniclers mention the Humber, the river Ouse or Northumbria (i.e. north of the Humber).   But other historians argue that these references derive from the single John of Worcester source, that John of Worcester was mistaken, and that a western landing site is more likely. They cite the short time between August and December 937 available to the allies to arrange a rendezvous and question whether the hazardous voyage round the north of the British Isles from Dublin would have been a practical proposition for Vikings. But these arguments have also been questioned on the grounds that, given the seafaring culture of the Vikings, Anlaf would have been unlikely to mount an attack on York, his military objective, by abandoning his fleet on the west coast and marching nearly 200 km across Anglo-Saxon territory,

Faced with this invasion, Aethelstan raised a huge army and headed north. The Anglo-Saxon Chronicle makes great play of the involvement of both West Saxons and Mercians in the ensuing clash of arms.

Accounts say the fighting at Brunanburh lasted all day. According to the Annals of Ulster the battle was ‘immense, lamentable and horrible, savagely fought’ while a contemporary poem concluded ‘never was there more slaughter on this island, never yet as many people killed before this… since from the east Angles and Saxons came up over the broad sea.’ Eventually the English gained the upper hand and the invaders broke and fled. Aethelstan and his army pursued them until nightfall... ‘hewing the fugitives grievously from behind with swords sharp from the grinding’. Anlaf made for the safety of his ships and sailed home over “Dingesmere” with what was left of his army, arriving back in Dublin in the spring of 938. Constantine escaped to Scotland and abdicated a few years later, Owain was probably killed in the battle. Aethelstan headed south in triumph. The English though also paid a heavy price for their victory; two of Aethelstan’s cousins had been slain, along with two bishops and possibly Thorold Skallagrimsson, and what the Annals of Ulster described as ‘a multitude’ of lesser men. The invaders’ losses were even higher. Five minor kings, seven Viking earls and one of Constantine’s sons were killed together with many of their followers.

Whilst Aethelstan had preserved the unity of his kingdom, his hold on parts of it remained fragile. When Aethelstan died in 939, Anlaf once again left for York, quickly established himself as king of Northumbria and then seized the East Midlands. Aethelstan’s successor Edmund took back control of the East Midlands and York after Anlaf died in 941 but after Edmund's own death in 945 York again briefly switched back to Viking control. In 954 its Scandinavian ruler, Eric, was killed by his rivals and Northumbria submitted to Eadred of Wessex.