Drang nach Osten ("Striving towards the East") is a term used in Germany's history that means the expansion of Germany, German states and German settlement, that led to the conquest of former Slavic and Baltic areas by Germany commencing during the Middle Ages until the end of World War II in 1945 when the Nazis were defeated by the Red Army of the Soviet Union.
Population growth during the High Middle Ages stimulated movement of peoples from the Rhenish, Flemish and Saxon territories of the Holy Roman Empire eastwards into the less-settled Baltic region and what is now Poland. These movements were supported by the German nobility and the medieval Church, and took place largely at the expense of non-Christian Baltic ethnic groups (see Northern Crusades).
The future German state of Prussia, named for one of the conquered peoples, had its roots largely in these movements. As the Middle Ages came to a close, the Teutonic Knights had assimilated (and forcibly converted) much of the southern Baltic coastlands, and a more or less stable frontier had formed between Prussia and the Catholic Polish kingdom. Russia and Sweden eventually conquered the lands taken by the Teutonic Knights in Estonia and Livonia.
This was the situation until the upheavals of the 19th and 20th, when German nationalists pointed to the movement towards the East as a proof of German "vitality". A definitive halt to the idea of the Drang nach Osten came during World War II, after Nazi Germany invaded the Soviet Union in 1941.
Nazi officials used it as grounds for the expulsion of 800,000 Poles from Warsaw to concentration camps after the defeat of the Warsaw Uprising in 1944, which caused 200,000 deaths. The city of Warsaw with millions of inhabitants was ordered to be completely demolished on Hitler's personal orders. Himmler stated that the Poles had been an obstacle to German Eastern expansion for the last 700 years, and that the aim was to remove that obstacle permanently.
Decisions made at the Potsdam conference in 1945, especially as relating to the Oder-Neisse line, rolled back the practical results of the later part of the Drang nach Osten and redesignated German territories within the approximate Germanic borders of the year 1200 AD. On the other hand, the historical Slavic lands of Sorbs (Wends, Polabian Slavs) between the Oder-Neisse line and the river Elbe remained within the territory of Germany.