Ostsiedlung / Hochmittelalterlicher Landesausbau (eastward expansion of the Holy Roman Empire during the Middle Ages)

Starting in about 1100, the borderlands (the so-called Marches) to the east of the Holy Roman Empire, an area known as Germania Slavica, in part populated by Slavic peoples such as the Sorbs, were settled by Germans and other peoples. The process came to an end around the start of the 1300s.

The Holy Roman Empire had previously extended as far as Magdeburg. Between 1000 and 1340, its population increased from 4 million to 11.4 million. Consequently, over the course of about 200 years, it extended eastwards into what would later be the German Democratic Republic (East Germany), into western Poland, and as far as the Baltic states. Koenigsberg (now Kaliningrad) was founded in 1286.  

Key figures at the start of this process were Graf Adolf von Schauneburg (1143), and Markgraf Albrecht der Baer (r. 1123-1170). Approximately 200 thousand people moved into these lands in the 1200s, and a comparable number in the 1300s. The Ostsiedlung proceeded in phases, the third of which included the future towns of Coelln and Berlin, initally founded in the mid 1200s. The current Nikolaikirche in the centre of what was East Berlin dates to this time. These and other towns would later amalgamate to form Berlin, which would become the capital first of Brandenburg, then of Prussia, and finally of the German Reich from 1870.  

The Ostsiedlung was accompanied by the work of the Teutonic orders, military and monastic in nature, who were very regretably responsible for the forced Christianisation of peoples in Central and Eastern Europe. While there were instances of expulsion or extermination, for the most part the incoming migrants assimilated with the existing population.