Alfred of Wessex died in 899 and was succeeded by his son Edward the Elder. Edward, and his brother-in-law Æthelred of (what was left of) Mercia, began a programme of expansion, building forts and towns on an Alfredian model. On Æthelred's death, his wife (Edward's sister) Æthelflæd ruled as "Lady of the Mercians" and continued expansion. It seems Edward had his son Æthelstan brought up in the Mercian court. On Edward's death, Æthelstan succeeded to the Mercian kingdom, and, after some uncertainty, Wessex.

Æthelstan continued the expansion of his father and aunt and was the first king to achieve direct rulership of what we would now consider England. The titles attributed to him in charters and on coins suggest a still more widespread dominance. His expansion aroused ill-feeling among the other kingdoms of Britain, and he defeated a combined Scottish-Viking army at the Battle of Brunanburh. However, the unification of England was not a certainty. Under Æthelstan's successors Edmund and Eadred the English kings repeatedly lost and regained control of Northumbria. Nevertheless, Edgar, who ruled the same expanse as Æthelstan, consolidated the kingdom, which remained united thereafter.