Warmatrix

Map of the movements of the Great Viking Army

The Great Viking Army was a Danish army of Vikings that ravaged England for 14 years, before being beaten in the Battle of Edington.
England had been raided by the Vikings since 793 CE. Most of the attacks were small, aiming at a few shiploads of booty. That changed in 865 CE, when a large force of Danes and Norwegians landed in East Anglia. The army probably numbered a few thousand fighters who had gained experience raiding the Frankish empire. Because resistance had stiffened there, they switched to England.
After landing, the army toured through East Anglia, Northumbria and Mercia, where they captured the city of Nottingham in 868 CE. Almost everywhere they went they were bought off with money. A combined army from Wessex and Mercia besieged them in Nottingham but achieved little and so the Mercians also paid the Vikings to leave. The invaders returned to Northumbria and East Anglia and beat the local king Edmund.
The Great Army was not a true army, but a collection of raiding bands under strong leaders, that struck where most loot could be gained. They preferred extortion over battle, but if forced to fight, displayed cunning and prowess. Sometimes parts split off on private enterprises; at other times they banded together for strength.
In 871 CE the army was reinforced from Scandinavia and then turned on Wessex. Æthelred and Alfred the Great defeated them at the battle of Ashdown, then lost at Meretun. A few months later Æthelred died and Alfred gained the throne. He also bought the Vikings off to buy time. The Great Army returned to Northumbria, where the local population had revolted against the puppet ruler Egbert whom the Vikings had set up. In 873 CE they forced the Mercians to pay them once more, but a year later attacked, drove the king into exile and conquered the kingdom.
Jarl Halfdan split off from the main force and campaigned in the north against Picts and Britons. Later he divided the lands of Northumbria among his men, a clear sign that the Vikings meant to stay. Frontier towns became fortified and were equipped with strong garrisons, while in the rear older warriors settled down as farmers.
In the meanwhile the southern half of the army under Guthrum returned to Wessex in 877 CE. They surprised Alfred and drove him to seek refuge in a marsh at Somerset, from which he mounted small counterattacks. But a year later, after the fields had been sown, Alfred struck back. He managed to defeat the Vikings once more at the Battle of Edington in 878 CE. In the peace settlement Guthrum agreed to be baptized and established himself as king in East Anglia. This ended the campaign of the Great Army.
Alfred learned his lesson and reorganized his forces. He set up a small standing army, created a strong fleet and a string of forts, the "burhs" at strategic locations. The latter blocked access routes, offered protection to the population in times of attack and the former allowed the English to strike back quickly. He reinstated a general levy, rather than relying on his nobles, some of whom were inclined to collaborate with the vikings.
The Battle of Edington and especially Alfred's reforms made sure that Wessex remained English. Subsequent Viking attacks were repulsed, but Northumbria, eastern Mercia and East Anglia were part of the Danelaw for quite some time. This formed the basis of the British half of the North Sea empire that Sweyn Forkbeard would later establish. Though the burden of Alfred's system on the economy was high, it reinforced English royal power. Because the defense later slackened again, it took successor kings a full century to make England whole again.