From as early as Ecgberht's time, the West Saxon order had faced viking defence; such pressures were central to Alfredian security. Viking activity was a dynamic political stimulus. Culturally, these Danish invaders need not have differed greatly from their opponents, yet their activities posed new challenges to native kingdoms. Amorphous social organization combined with extreme mobility by land and sea to exploit weaknesses in local defence. Both war-bands and armies regularly divided resistance through convenient alliance; opportunism continued in a flexibility of strategic methods, all involving material extraction. Dominated by the redistribution of moveable wealth, viking needs tapped plunder, ransoming and tribute. Initially pursued by piecemeal raiding, from mid-century such income found exploitation by larger armies, borne by substantial fleets of well over 100 ships; corroborated by independent annals, such figures support armies numbering thousands rather than hundreds. Viking extraction was probably critical in elevating the goal of outright conquest, seemingly pursued with consistency by the ‘Great Army’ from 865. Whatever the case, its successes were swift and comprehensive, Northumbria falling in 867 and East Anglia in 869; by 877 eastern Mercia also lay under viking control. Raiding poachers had turned legitimate gamekeepers: nowhere in Europe was this transformation more effective or extreme.

Only the West Saxon kingdom proved capable of surviving this campaign, narrowly reversing a partial submission to viking rule in 878.