The Great Viking Army or Great Danish Army, known by the Anglo-Saxons as the Great Heathen Army, was a coalition of Norse warriors, originating from Denmark and Norway (and possibly also from Sweden) who came together under a unified command to invade the four Anglo-Saxon kingdoms that constituted England in AD 865.
Since the late 8th century, the Vikings had settled for mainly ”hit-and-run” raids on centres of wealth, such as monasteries. However, the intent of the Great Army was different. It was much larger than the usual raiding party and its purpose was to conquer.
The name Great Heathen Army is derived from the Anglo-Saxon Chronicle of 865. Legend has it that the force was led by three sons of Ragnar Lodbrok. The campaign of invasion and conquest against the four remaining Anglo-Saxon kingdoms lasted 14 years. Unlike many of the Scandinavian raiding armies of the period, surviving sources give no firm indication of its numbers, but it was clearly amongst the largest forces of its kind.
The invaders initially landed in East Anglia, where the king provided them with horses for their campaign in return for peace. They spent the winter of 865–66 at Thetford, before marching north to capture York in November 866. York had been founded as the Roman legionary fortress of Eboracum and revived as the Anglo-Saxon trading port of Eoforwic.
During 868, the army marched deep into Mercia and wintered in Nottingham, where it was besieged by a joint force from the kingdoms of Mercia and Wessex. With no progress being made, the Mercians agreed to terms with the Viking army, which moved back to York for the winter of 869–70. In 870, the Great Army returned to East Anglia, conquering it and killing its king. The army moved to winter quarters in Thetford. In 871, the Vikings moved on to Wessex, where Alfred the Great was content to pay them to leave. The army then marched to London to overwinter in 872 before moving back to Northumbria in 873. It again returned to Mercia, conquering it in 874 and overwintering at Repton on the River Trent. By this time, only the kingdom of Wessex had not been conquered by the invading Vikings. It was towards the end of 875 when the army started their second invasion of Wessex. After a few setbacks, Alfred the Great defeated the Vikings at the Battle of Edington, and a treaty was agreed upon, whereby the Vikings were able to remain in control of much of northern and eastern England.