Halfdan Ragnarsson (; or Healfdene; ; died 877) was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865. One of six sons of Ragnar Lodbrok named in Norse sagas, Halfdan's brothers or half-brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye, Ubba and Hvitserk. Because Halfdan is not mentioned in any source that mentions Hvitserk, some scholars have suggested that they are the same individual – a possibility reinforced by the fact that Halfdan was a relatively common name among Vikings and Hvitserk "white shirt" may have been an epithet or nickname that distinguished Halfdan from other men by the same name. Halfdan was the first Viking King of Northumbria and a pretender to the throne of Kingdom of Dublin. It is also possible he was for a time co-ruler of Denmark with his brother Sigurd Snake-in-the-eye, because Frankish sources mention certain Sigfred and Halfdan as rulers in 873. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim. Biography Halfdan was one of the leaders of the Great Heathen Army which invaded the Anglo-Saxon kingdom of East Anglia in 865. According to the Norse sagas this invasion was organised by the sons of Ragnar Lodbrok (Halfdan being one of them), to wreak revenge against Ælla of Northumbria. Ælla had supposedly had Ragnar executed in 865 by throwing him in a snake pit, but the historicity of this explanation is unknown. The invaders are usually identified as Danes, although the tenth-century churchman Asser stated that the invaders came "de Danubia", which translates as "from the Danube"; the fact that the Danube is located in what was known in Latin as Dacia suggests that Asser actually intended Dania, a Latin term for Denmark. In the autumn of 865 the Great Heathen Army landed in East Anglia, where they remained over the winter and secured horses. The following year the army moved northwards and invaded Northumbria, which was at that time in the middle of a civil war between Ælla and Osberht, opposing claimants for the Northumbrian throne. Late in 866 the army conquered the rich Northumbrian settlement of York. The following year Ælla and Osberht made an alliance to retake the town. The attack was defeated, and both of them fell in the battle. With no obvious leader, Northumbrian resistance was crushed and