How was Alfred able to defeat the Vikings

Question: Why was Alfred Able To Defeat The Vikings? In the autumn of 856, Danish Viking ships sailed to England and invaded what is now Norfolk in East Anglia. By Easter of 878, the Danes held control of all of East Anglia, Mercia, most of Wessex (all save Hampshire, Wiltshire and Somerset) and had set up a puppet ruler in Northumbria. The invaders had destroyed or overrun almost everything that had been of the four great Anglo-Saxon …

…