The arrival of the "Great Army" late in 865 signalled a new phase of Viking activity in England. The army, under the joint leadership of Halfdan and Ivar the Boneless, over wintered in Thetford.

In 866, having intimidated the East-Anglians into providing horses for them, the Great Army set about a series of campaigns that successfully overcame every Anglo-Saxon kingdom with the exception of Wessex.

A turning point in these activities occurred in 874 when the army split after leaving their winter quarters in Repton. Halfdan took his section of the army into Northumbria, where they settled; Guthrum and his forces advanced upon Cambridge. If this split had not occurred, it is exceeding unlikely that Wessex, under the leadership of King Alfred, could has withstood the Vikings.

As it was, however, Alfred won a decisive battle in 878 at Edgington, which led to the Treaty of Wedmore and the partitioning of the country into Wessex and Anglo-Saxon Mercia on the one side of the boundary, and the Danelaw on the other.