Halfdan Ragnarsson in the context of Ubba

The Great Heathen Army, also known as the Viking Great Army, was a coalition of Scandinavian warriors who invaded England in AD 865. Since the late 8th century, the Vikings had been engaging in raids on centres of wealth, such as monasteries. The Great Heathen Army was much larger and aimed to conquer and occupy the four kingdoms of East Anglia, Northumbria, Mercia and Wessex.

The name Great Heathen Army is derived from the Anglo-Saxon Chronicle. The force was led by three of the five sons of the semi-legendary Ragnar Lodbrok, including Halfdan Ragnarsson, Ivar the Boneless and Ubba. The campaign of invasion and conquest against the Anglo-Saxon kingdoms lasted 14 years. Surviving sources give no firm indication of its numbers, but it was described as amongst the largest forces of its kind.

The Battle of Reading was a victory for a Danish Viking army over a West Saxon force on or about 4 January 871 at Reading in Berkshire. The Vikings were led by Bagsecg and Halfdan Ragnarsson and the West Saxons by King Æthelred and his brother, the future King Alfred the Great. It was the second of a series of battles that took place following an invasion of Wessex by the Danish army in December 870.

The Battle of Ashdown was a West Saxon victory over a Danish Viking army on about 8 January 871. The location of Ashdown is not known, but may be Kingstanding Hill in Berkshire. Other writers place the battle near Starveall, a short distance north of the village of Aldworth and south east of Lowbury Hill.

The West Saxons were led by King Æthelred and his younger brother, the future King Alfred the Great, while the Viking commanders were Bagsecg and Halfdan. The battle is described in the Anglo-Saxon Chronicle and Asser's Life of King Alfred.

Ivar the Boneless (Old Norse: Ívarr hinn Beinlausi [ˈiːˌwɑrː ˈhinː ˈbɛinˌlɔuse]; died c. 873), also known as Ivar Ragnarsson, was a Viking leader who led invasions into England and Ireland. According to the Tale of Ragnarr Loðbrók, he was the son of Aslaug and her husband Ragnar Loðbrok, and was the brother of Björn Ironside, Halvdan (or Hvitserk), Sigurd Snake-in-the-Eye, and Ragnvald. However, it is not known whether this is historically accurate. Ivar is sometimes regarded as the same person as Ímar, a Viking king of Dublin between 870 and 873.

It is unclear why Ivar acquired the nickname "boneless". Some sagas claim that he was born with a skeletal condition which left him unable to walk, while others suggest that he was merely impotent.