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 a part of its territory to Poland. Previously, the German Empire 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. In earlier centuries these territories were part of the Polish-Lithuanian Commonwealth. They were annexed to the Kingdom of Prussia, as a result of the tripartite partition of the Polish-Lithuanian Commonwealth by the Kingdom Prussia, the Holy Roman Empire and the Russian Empire.
Most of the ethnically-German part of the population of these provinces, alongside most of the German minority population in what was Poland before the Second World War (741,000 people), either fled before the Red Army to central and western Germany or were forcibly resettled later. It should be noted here that German citizens of Polish descent and ethically-Polish residents of these regions and other parts of Germany faced systematic discrimination, internment in concentration camps, property-confiscations, forced-resettlement and expulsion under the Nazi administration.