In the time of Charlemagne the Elbe River marked a pretty clear boundary between the Slavic population to the east and the Germanic to the west. There were many Slavs west of the Elbe, but no Germans east of it. There had been a time when Germans occupied large portions of eastern Europe, but for one reason or another they gradually became concentrated toward the west, while Slavic peoples pushed in to fill the vacated territory. Under Charlemagne and his successors we can discern the earlier stages of a movement of reaction which has gone on in later times until the political map of all north central Europe has been remodeled. During the ninth, tenth, and eleventh centuries large portions of the "sphere of influence" (to use a modern phrase) which Charlemagne had created eastward from the Elbe were converted into German principalities and dependencies. German colonists pushed down the Danube, well toward the Black Sea, along the Baltic, past the Oder and toward the Vistula, and up the Oder into the heart of modern Poland. The Slavic population was slowly brought under subjection, Christianized, and to a certain extent Germanized. In the tenth century Henry I. (919-936) began a fresh forward movement against the Slavs, or Wends, as the Germans called them. Magdeburg, on the Elbe, was established as the chief base of operations. The work was kept up by Henry's son, Otto I. (936-973), but under his grandson, Otto II. (973-983), a large part of what had been gained was lost for a time through a Slavic revolt called out by the Emperor's preoccupation with affairs in Italy. Thereafter for a century the Slavs were allowed perforce to enjoy their earlier independence, and upon more than one occasion they were able to assume the aggressive against their would-be conquerors. In 1066 the city of Hamburg, on the lower Elbe, was attacked and almost totally destroyed. The imperial power was fast declining and the Franconian sovereigns had little time left from their domestic conflicts and quarrels with the papacy to carry on a contest on the east.