Focus on environment: trade

"It did not take long, however, for Scandinavian merchant adventurers to become aware of a particular important fact of east European geography. While some of the rivers of European Russia flow north into the Baltic, others drain south into the Black and Caspian Seas. The whole area is so flat, moreover, that the headwaters of both north- and south-flowing rivers lie extraordinarily close together. A combination of tributaries, especially the west-east flowing River Oka, combined with carefully reconnoitered [observed, studied] portages, where ships were dragged usually on rollers from one set of connected river systems to another, allowed access to the Black and Caspian Seas via two major routes, the Dnieper [River] and the Volga [River].

The Volga route led straight to the Caspian Sea and the economically developed world of the Islamic Caliphs, by this date based on the Abbasid capital of Baghdad. There the taxes of a vast empire, stretching from the Atlantic to India, were being consumed by a court of stupendous magnificence. Here was the real center of demand for merchants with luxury goods to sell.... The Dnieper route, by contrast, was far more difficult, involving some awkward rapids around which boats had to be carried, and led out to the Black Sea...and the more natural trade axis led to Constantinople... Byzantium was a sadly reduced power from its glory days under Justinian, and the Islamic Caliphs and their court grandees represented a far richer market for the luxury goods that the Scandinavians had to offer."