Of all of the history and lore that has come out what we sometimes call The Early Middle Ages (circa 476 to 1000 A.D.), some of the most interesting and exciting are the Vikings
The Viking Age (793 - 1066 A.D.) saw a great movement of the peoples of Norway, Sweden, and Denmark throughout Europe to Russia and across the seas to North America (establishing colonies in Iceland and Greenland in the process):
Why they left when they did it is largely beyond the scope of discussion we have been engaging in, but in short it appears it was a combination of swelling populations and decreasing land availability in the lands of Scandinavia and, oddly enough, by the destruction of the Frisian Pirates by Charlemagne, which - much like Byzantium later in history - removed the buffer between Europe and those that would raid it. The seas to much of Europe at this time (including Anglo-Saxon England) were more like moats; to the Vikings they were boulevards, allowing travel almost at will.
The first recorded Viking attack on Anglo-Saxon England (we assume; no one questioned them) was in 787 A.D. when three ships appeared on the Dorset coast of Wessex. The local royal official, the port reeve, came to the shore to question their business. Their response was to kill the port reeve and sack the town.
Now the English Heptarchy had two issues: jostling among themselves and trying to defend themselves against the increasing tempo of the Viking raids.
Nicolle, David: Arthur And The Anglo-Saxon Wars. Osprey Publishing: Hong Kong, 1984