The border between eastern Germany and Poland formed from the Oder-Neisse river system and extending from the Baltic Sea to the Czech Republic. In the closing stages of the Second World War, as the Allied Powers debated the future make-up of a post-Nazi Europe, the reconstruction of Poland, and therefore also of Germany, became of central importance. Poland was liberated by Soviet forces that went on to occupy Berlin in 1945, putting the Soviet Union in a commanding position in negotiations over Poland's future. Since the country was losing substantial eastern territories to the Soviet Union itself, the new communist authorities in Poland pressed for territory in the west at the expense of defeated Germany. The Soviet leadership consequently proposed the course of the lower Oder river, and its tributary the Neisse, as a natural frontier, pushing the new Poland far into historically German lands. Initially the idea was opposed by the Western allies, seeking to limit the consequent population movements and national upheaval for Germany However, the presence of Soviet troops on the ground, and the desire of the Western leaders to be accommodating towards Stalin on this issue, led the Allies to agree to the Oder-Neisse proposal in the Yalta and Potsdam Agreements. Millions of Germans were forcibly deported from the annexed territory.
The post-1945 borders were recognized by the newly-established East German state in 1950. West Germany, however, continued to regard them as no more than a temporary administrative border until 1971, when a change in stance on policy towards the East was marked by recognition of the enduring status of the Oder-Neisse line. At the time of German reunification in 1990, the Federal Republic of Germany moved quickly to attest the legitimacy of the Oder-Neisse line as Poland's inviolable western border. This was confirmed in the German-Polish Treaty signed in Warsaw on 14 November 1990.