A number of theories have been advanced to explain the Viking expansion. The establishment of the Holy Roman Empire by Charlemagne, with the ruthless subjugation and forced Christianization of the Danes’ Saxon neighbours, was clearly influential. The Danevirke, a huge defensive fortification across the neck of the Danish peninsula, was reinforced in the early 800s to avert this threat. But the notion of a seaborne counteroffensive against Christendom hardly explains Swedish Viking expansion into Russia, or the Norwegian colonization of Iceland and Greenland. More significant, perhaps, was the decline in commerce caused by Islamic expansion; the Vikings set out in search of new markets, and when their naval and military supremacy became evident, turned to conquest. While creating turmoil abroad, the Scandinavian kingdoms enjoyed unusual stability in the Viking period. They were never invaded and had a succession of long reigning monarchs buoyed by the largesse from their far-ranging subjects.
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