Oder-Neisse line

The Oder–Neisse line (Polish: granica na Odrze i Nysie Łużyckiej, German: Oder-Neiße-Grenze) is the international border between Germany and Poland. It was drawn at the Potsdam Conference in the aftermath of the Second World War and is primarily delineated along the Oder and Lusatian Neisse rivers in Central Europe, meeting the Baltic Sea to the north, just west of the Polish seaports of Szczecin and Świnoujście (German: Stettin and Swinemünde). All prewar German territory east of the line and within the 1937 German boundaries (23.8% of the former Weimar Republic) were placed under International Law Administrative status, with most of it being made part of newly-Communist Poland. The small remainder, consisting of the territory surrounding the German city of Königsberg (now renamed Kaliningrad, in honour of Soviet head of state Mikhail Kalinin) in northern East Prussia, was allocated to the Soviet Union (as Kaliningrad Oblast of the Russian SFSR, today the Russian Federation) after the war (pending the final World War II peace treaty for Germany). The vast majority of the native German population in these territories fled, or was killed or expelled by force. The Oder–Neisse line marked the border between the German Democratic Republic (East Germany) and Poland from 1950 to 1990. East Germany confirmed the border with Poland in 1950, while West Germany, after a period of refusal, finally accepted the border (with reservations) in 1970. In 1990 the newly reunified Germany and the Republic of Poland signed a treaty recognizing it as their border.

...
Wikipedia