The Oder-Neisse line (named after the Oder and Neisse rivers) is the border between Germany and Poland since the end of the Second World War. As a result of the defeat Germany lost another huge part of its territory to Poland after it already had to cede the former Prussian provinces of Posen and Westpreußen as well as parts of upper Silesia to the newly founded Polish state after the First World War in 1919.
The population of the eastern German provinces as well as the German minority population in what was Poland before the Second World War (12 million) either fled before the Russian army to central and western Germany or were later driven out of their homeland by force of the Polish militia who entered immediately after the Russian army.
As Poland annexed the German provinces and expatriated the native population, Polish people occupied those provinces and took over land and property of the expatriated Germans. About 1.5 million of the new inhabitants had been driven out of their own homeland in eastern Poland/Western Ukraine, Belarus, as the U.S.S.R. was not willing to give back that part of 1922-1939 Poland which it had annexed in 1939.