Æthelstan or Athelstan was the son of King Edward the Elder and his first wife, Ecgwynn. During his own reign Æthelstan’s contemporaries regarded him as ‘one of the most excellent and illustrious among the earthly kings of our own day’. After Æthelstan's coronation, in January 926, he arranged for his only full sister to marry Sihtric. The two kings agreed not to invade each other's territories or to support each other's enemies. The following year Sihtric died, and Æthelstan seized the chance to invade. At Eamont on 12 July 927, King Constantine of Scotland, King Hywel Dda of Deheubarth, Ealdred of Bamburgh, and King Owain of Strathclyde (or Morgan ap Owain of Gwent)[e] accepted Æthelstan's overlordship. He then went to the Welsh border and forced the Welsh princes to accept his authority and pay an unusually high level of tribute. A more permanent result of the meeting was an agreement to fix the border between England and Wales

Victory over combined force of Scots, Welsh and Vikings at the Battle of Brunanburh in 937 confirmed his prestige. 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. Æthelstan's defeat of the combined Norse-Celtic force facing him confirmed England as a fully unified kingdom. However, he was militarily weakened and the battle effectively forced all the kingdoms of the British Isles to consolidate in the positions they occupy today. He claimed the title Rex totius Britanniae, King of the Whole of Britain. In one manuscript dedication he is even styled ‘’basileus et curagulus’’, the title of Byzantine emperors. The contemporary French chronicler Flodoard described him as “the king from overseas”. According to William of Malmesbury, the gifts given to Æthelstan included spices, jewels, many swift horses, a crown of solid gold, the sword of Constantine the Great, Charlemagne's lance and a piece of the Crown of Thorns to emphasize his greatness even more.