The Viking Era

Poul Anderson, "THE NORSE" IN Poul Anderson & Mildred Downey Broxon, The Demon Of Scattery (New York, 1980), pp. 200-207.

(The title page of this edition says "SF ace books," which is inaccurate.)

According to Anderson's historical note:

The "Vikings" were not a people and the word should not be capitalized. The earliest recorded viking raids on England and Ireland occurred in the late eighth century. (Thus, Hrolf Kraki, in the mid-sixth century, was much earlier. See "The History of Hrolf Kraki: a Foreword by Poul Anderson" IN Poul Anderson, Hrolf Kraki's Saga (New York, 1973), pp. xvii-xxi AT p. xviii.)

The viking period lasted for about three centuries:

England, Ireland, France, Germany and the Low Countries were attacked repeatedly;
in 845, Paris and Hamburg were captured;
at least one expedition plundered in the Mediterranean;
Finns, Lapps and Balts were attacked but had no one to record it;
the attackers were from Scandinavia or from their colonies, including Iceland;
the probable causes were population pressure, ambition and greed;
some bands would sow, go in viking and return for harvest;
"viking" is probably derived from "...'vik,' meaning a narrow bay..." (p. 202), because raiders would wait in such a bay to attack passing cargo ships;
"viking" rhymed with "seeking";
kings and jarls discouraged such attacks at home but did not object to attacks on foreign countries;
some huge fleets wintered abroad, then colonized in England, Ireland, Normandy etc.