What caused the Scandinavian - and in particular the Viking migrations, is not altogether known. Until their conversion to Christianity, the Scandinavians were illiterate. What is known about them comes from fragmentary archaeological evidence and what the peoples they invaded said about them, which was extremely hostile. They were heathen robbers bent on pillaging churches and homes, looting monasteries and cathedrals, raping nuns and in some cases, seizing agricultural land from peaceful farmers. Historians speculate that the Scandinavians were impelled to move southward by overpopulation, deteriorating climate and food supply, political squabbles in which the losers were driven overseas, or a combination of all these factors.
Some historians take a more positive view of the Vikings.
They were as interested in trade as in war and looting. They were attracted by the wealth and trading opportunities of Constantinople, which drew them to attempt to traverse the river systems of Russia. The Vikings stood in awe of the rich farmlands of northern France and southern England and sought access to the markets created by the abbeys and towns. In Latin writings, the Vikings appear as fierce heathen warriors and ruthless pillagers from about 850 - 1000. In time the Vikings became Christian lords, farmers and merchants, just like everyone else.