Around 850 CE, the Vikings stopped looking at England and Ireland as places to raid or seek mercenary work, but as places to colonize and settle. In 865, the Vikings banded together and formed what the Saxons called “The Great Heathen Army”. Under the leadership of Ivar the Boneless, Halfdan,Ubba, Sigurd Snake-Eye, and Bjorn Ironside i.e. the sons of semi-legendary Viking warlord Ragnar Lothbrok, the Great Heathen Army conquered the Anglo Saxon kingdoms of Northumbria, Mercia, and East Anglia, and formed the Danelaw, where Saxon law held no sway.
In 871, only the kingdom of Wessex still stood against the Danelaw, and in early January the Vikings invaded. On 4 January, the Saxon ealdorman Aethelwulf defeated a large raiding party at the Battle of Englefield which forced the Viking army to establish a camp at Reading to reorganize. On 6 January Aethelwulf was joined by the main West Saxon army led by King Aethelred and his younger brother Alfred. Aethelred immediately attacked the camp at Reading, where Halfdan Lothbrok’s men ambushed and defeated the Saxons, and forced their retreat.
Aethelred retreated to the Ashdown fields closely pursued by the entire Viking host. On the morning of 9 January 871, the West Saxon army was arrayed on a ridge while the Vikings approached in a much disorganized manner. Not waiting for his brother Aethelred to finish prayers, young Alfred seized the moment and ordered the Saxon army to attack. The hasty Viking shield wall held for a time but the disciplined and well led Saxon shield wall eventually broke them. Ashdown was a stunning and much needed West Saxon tactical victory but it was hardly decisive and the war would continue for many years. (Neither side had cavalry, the Combat Arm of Decision, and could not pursue and destroy a retreating army after a victory)
However, the Battle of Ashdown would cement Alfred’s leadership of the West Saxon army. In April, King Aethelred died and Alfred would be crowned King of Wessex despite Aethelred’s two sons. Over the next ten years Alfred would lose all of Wessex to the Great Heathen Army… but then subsequently regain it, reconquer the Danelaw, and unify the disparate Anglo Saxon kingdoms into greater England. For this he would forever be known as King Alfred the Great.