The Great Heathen Army, also called the Great Viking Army or Great Danish Army, was an army of Vikings that raided and conquered much of England in the late 9th century, coming from Denmark.
The army was exceptionally large by the standards of the time and probably included several thousand fighters. It was not an army under unified leadership, but rather many groups, most of which proceeded separately and sometimes fought each other.
The English name Great Heathen Army comes from the Anglo-Saxon Chronicle. The most important historical knowledge about this army comes from this work.
The origins of the army are seen in a group of Viking warriors who attacked Paris in 845 and settled in the region from 850, repeatedly sacking Rouen.
Late in 865, they landed in East Anglia. Under the command of the brothers Halfdan Ragnarsson and Ivar the Boneless, and with the support of their brother Ubba Ragnarsson, the army attempted to conquer England and establish settlements. The trigger may have been the execution of their father Ragnar Lothbrok by Ælle in 865.
In 866 they conquered the Anglo-Saxon kingdom of Northumbria and in 870 East Anglia. In 871 they received reinforcements in the form of the Great Summer Army from Scandinavia.
The army, thus further increased, also conquered Mercia in 874. At the same time, the first settlements began in the conquered territories, and a second settlement push followed in 877.
Halfdan went north to war with the Picts, while Guthrum, who had come with the Great Heathen Army, remained in the south as army commander.
Troop reinforcements came again in 876, and so they defeated Alfred the Great of Wessex at the Battle of Wareham. Alfred won the Battle of Eddington in May 878 and made peace in the Treaty of Wedmore.
A part of the defeated Vikings retreated to continental Europe and moved their raids to the coastal region of the English Channel, northern France and Flanders. As a result, major raids by the Vikings also occurred for the first time in the Rhineland.
The settlers who remained in England founded the Kingdom of Jórvík (York), which lasted (with interruptions) until 950 and was part of the Danelaw.
The Great Heathen Army, also called the Great Viking Army or Great Danish Army, was an army of Vikings that raided and conquered much of England in the late 9th century, coming from Denmark. 
