German eastward expansion (Ostsiedlung)

Ostsiedlung (German: Settlement in the East), also known as German eastward expansion, refers to the medieval eastward migration and settlement of Germans from modern day Western and Central Germany into regions like the Baltic and modern day Poland inhabited by Baltic peoples and the Slavs Even though first settlements led by Franks and Bavarii followed the defeat of the Avars in the early 9th century, and other campaigns by Holy Roman Emperors allowed migration, the beginning of a continuous Ostsiedlung is mainly dated to around the 12th century.

In German scholarship, it refers especially to the reassertion of Saxon authority over Sorbian or Wendish areas, especially Brandenburg by Albert the Bear in the 1150s.
The advent of the crusading Teutonic Order, which had been invited in the 1220s to support the conquest of pagans like Old Prussians, accelerated settlement along the Baltic coast.

The High Middle Ages saw a rapidly increasing population of Europe which could not be fed by "great clearances" of forests and marshes alone. During the Ostsiedlung, Germans settled east of the Elbe and Saale rivers, regions largely inhabited by Polabian Slavs. Likewise, in Styria and Carinthia, German communities took form in areas inhabited by Slovenians. The emigration of the inhabitants from the Valais valley in Switzerland to the areas that had been settled before by the Romans had to some extent the same preconditions as the colonisation of the East, for example, Romania.

In the middle of the 14th century, the settling progress slowed as a result of the Black Death; in addition, the most arable and promising regions were largely occupied.

Local Slavic leaders in late Medieval Pomerania and Silesia continued inviting German settlers to their territories. As late as the 18th century, well after the Thirty Years War had reduced Germany's population by a third, some Germans followed invitations to settle as far away as the Volga River.

In the 19th century, recognition of this complex phenomenon coupled with the rise of nationalism in Germany led to the concepts of Pan-Germanism and Drang nach Osten, which in part gave rise to the concept of Lebensraum. During and after World War II, Germans were expelled east of the Oder-Neisse line, partially reverting the results of the Ostsiedlung.