The Viking Age in England began with raids, the earliest recorded anywhere in Europe, on Portland (c789) and the monastery of Lindisfarne (793). A reference in the 792 charter of Offa, King of Mercia, to military service against 'pagans' may be evidence of other unrecorded raids around this time. The Anglo-Saxons prevented the Viking raiders penetrating inland by blocking rivers with bridges, and raiding remained small in scale until c835, when larger fleets began arriving and won some major victories, killing, for example, Raedwulf, king of Northumbria, in battle in 844. Attacks escalated further in 850 when a fleet, said to number 350 ships, established a winter camp on the Isle of Thanet near the mouth of the River Thames. Nevertheless, England suffered less severely from Viking attacks in this period than Francia or Ireland.
The decisive Viking intervention in England began with the arrival of an exceptionally large Danish army under Ivar Halfdan in the kingdom of East Anglia in 865. The Anglo-Saxon kingdoms did not cooperate effectively against the invaders, and by 876 the Danes had occupied East Anglia, eastern Mercia and southern Northumbria. Only the kingdom of Wessex, under Alfred the Great, successfully resisted Danish attacks.
Conquest was followed by widespread Danish settlement in eastern England and the foundation of small kingdoms, the longest of which was based in York. This Danish-settled area later came to be known as the Danelaw because of its distinctive Scandinavian-influenced customs. Around 900 Norse settlers, many of them from Ireland, began to settle quietly in north-west England.
Once the Vikings settled down, they lost their main military advantage - their mobility - and they became vulnerable to counter-attack. Alfred's successor, Edward the Elder, aided by his sister Aethelflaed, the ruler of Mercia, conquered the Danish-controlled areas south of the River Humber in 912-918, while the Danish kingdom of York came under Norse control in 919. Edward's son, Athelstan, captured York in 927, becoming the first king to rule over all of England. Not all Anglo-Saxons welcomed the rule of Wessex; some preferred the Danes and even fought for them. After Athelstan's death in 939 the Norse recaptured York, but could never hold it securely, and its last Viking king, Erik Bloodaxe, was overthrown and killed in 954.
Notes from Encyclopaedia of the Viking Age by John Haywood. Thames & Hudson 2000. p64