Excerpts from Books and Wikipedia
"It was the first recorded raid by Viking pirates from Denmark and Norway, who had crossed the North Sea in search of plunder. In 794 another monastery in Northumbria was burned, and the monastery of St. Columba on the Scottish island of Iona was attacked a year later. From then on, the Viking longships arrived every summer, raiding around the coasts of England, Scotland, and Ireland before returning to Scandinavia for the winter. By the end of the decade, the ships were venturing as far as western France." [1001 Days] "The Vikings were the most successful fighting men in Europe for much of the ninth, tenth and into the eleventh centuries, for Ireland in the west to Kiev in the east. They were vigorously recruited by the Byzantines." [Cambridge Illustrated History of Warfare, p. 76]