There were some small glimmers of bravery and success. In the 840s Aþelstan, the under-king of Kent, and Ealhhere, his ealdorman, defeated a Danish force in a sea battle off Sandwich. In 845 the combined fyrds (mandatory levies) of Dorset and Somerset defeated a Danish army at the mouth of the Parrett. And in 851 King Æþelwulf of Wessex defeated an army of 350 ships' crews that had stormed Canterbury and London and put King Beorhtwulf of Mercia to flight.

This cemented Æþelwulf's position as the pre-eminent English king, although it was his father Egbert who had annexed Kent, Sussex, Surrey, and Essex.

The accession of his son Æþelred in 865 coincided with a dramatic change in viking activity in England. Instead of isolated raiders, in late 865, the Great Army sailed across the English Channel and arrived in East Anglia. Under the command of Halfdan Ragnarsson and Ivar the Boneless, with the support of Ubbe Ragnarsson, it aimed to conquer and settle England. This may have been in response to the death of their father, Ragnar Lodbrok (Ragnarr Loðbrók), at the hands of Ælle II of Northumbria earlier that year (supposedly in a snake-pit like Gunnar), but Stenton dismisses that as a legend. In any event, in late 866 they occupied York and conquered Northumbria, followed after another attack on York by East Anglia in 870, killing King Edmund, who rapidly came to be considered a saint. In fall or early winter that year, they camped in Wessex, near Reading, and Æþelred and his brother Alfred attacked them twice, the second time putting them to flight. (Æþelred famously refused to join the second battle, the Battle of Ashdown, until he had finished praying, which may well have given his attack extra force.) In April 871, the Great Summer Army arrived from Scandinavia. This reinforced the Great Heathen Army, enabling it in 874 to conquer Mercia. The same year, a considerable section settled in the conquered territories, followed by a further section in 877. Halfdan moved north to attack the Picts, while Guðrum emerged as the war leader in the south, and in 876 they were joined by further forces and won the Battle of Wareham.

The settlers from the army formed the Kingdom of York (Norse Jorvík), which survived with several interruptions until the 950s.

However, Alfred had succeeded his brother as king in mid-April 871, and assisted by the splitting off of the settlers from the Danish force, fought back. Although at one point he was hiding in defeat on the Isle of Athelney (where he took refuge with a cotter and his wife but disgraced himself by letting the oatcakes burn because he was too busy thinking about military strategy to watch them as the lady had told him to; she gave the king a piece of her mind), he eventually won victory at the Battle of Edington in 878, achieving the Treaty of Wedmore. Guðrum was baptized and pronounced king of East Anglia and led his army peacefully there to settle, and after the threat of further viking incursions plus an assessment of how many vikings were now settled in East Anglia, Mercia, and what is now Yorkshire, in 886 Alfred led a force to recapture London. Its walls were repaired and a garrison stationed there. It was the first time viking-occupied territory had been recaptured; it made the city a defensible center of English resistance against attack; and it established Alfred as king of what remained of Anglo-Saxon England. The viking raids had served to unite the country.

It was a country effectively cut in half: in 886 Alfred and Guðrum, as equals, made a treaty defining all England east of Watling Street as under Danish law - the Danelaw. Beyond that line, for many years the language spoken was Norse and the religion was heathenry. While on the Anglo-Saxon side of the line the country was united increasingly closely, English law would now record different terminology and different legal remedies for the Danelaw.

The settlement of the war with the Great Army did not end the viking threat. In fall 892 a large Danish army, which had been defeated in the Low Countries the previous year by Arnulf, king of the East Franks, assembled at Boulogne and crossed to England. Alfred had taken oaths from the Northumbrian Danes that they would not make common cause with it, and both oaths and hostages from the East Anglian Danes, but they nonetheless did so as soon as it landed. Alfred decided that the only way to reduce the vikings' power was to fight back at sea, and accordingly founded the English Navy. According to the Anglo-Saxon Chronicle he designed ships twice as large as the vikings' own, with 60 or more oars. He also reformed the fyrd (the mandatory national levy) by making provision for half the ceorls to stay at home to tend the fields while the other half served, which made for more willing and less exhausted troops. By some reports, he made greater use of þegns as a more disciplined fighting force. And he started to set up a system of forts no more than 20 miles apart in southern England, using the remains of Roman fortifications where possible. At his death in 899 the country was still beset by raids from the Danelaw and in collusion with the settlers there, and the system of fortifications was not complete: the story goes that when the invaders from Boulogne landed in 892, the only resistance they met was from a few peasants sitting in a half-built fort. But he had transformed the country to meet the viking threat and implemented several revolutionary solutions (both peaceful and warlike) that would enable it to survive.