In 865 AD, England was invaded by the Great Heathen Army. The Great Viking Army, as it was also known, was made up of a coalition of Scandinavian warriors mainly from Denmark and, legend has it, four of the five sons of Ragnar Lothbrok, including Halfdan Ragnarsson, Ivar the Boneless, Bjorn Ironeside and Ubba.
This was an army that would change Britain forever. It would lay siege to towns, attack monasteries and kill kings - the most famous of course being St Edmund, who was brutally beheaded by the Norse warriors in 869 AD.
Dan Snow was recently joined by bioarchaeologist and Viking specialist Cat Jarman on a road trip across England to retrace the path of this conquering Viking Army.