When it became clear that the Germans would lose World war II in 1943 discussions began on what the world would look like after the fall of the Nazis. In particular the Polish border was of some concern. Being the initial flash point for the war, and the longest occupied nation, Poland felt entitled to German land as compensation for their oppression, the Western allies also strongly supported this opinion. Poland’s eastern border had been annexed by the Soviet Union in 1939 per the Molotov-Ribbentrop pact and Poland was not keen on forgetting it. Despite this situation there was little Poland could do to change their border to the east. Understanding that this resentment may lead to problems in Stalin’s post-war sphere Stalin proposed moving Poland’s western border west substantially, as seen in the diagram above.
At the Tehran and Yalta conferences, of 1943 and 1945 respectively, The Eastern and western borders of Poland were debated. The United States and Britain, recognizing the wrong done to Poland by the Soviets, petitioned Stalin for the eastern borders of Poland to be altered so as to include the majority Polish city of Lwow, now in modern day Ukraine. Stalin refused but instead offered Poland generous compensation in the form of parts of East Prussia, Silesia, and all parts of Germany up to the Oder and eastern Neisse rivers. It was expected that all Germans who lived in these areas were to be expelled back to postwar Germany.
At Yalta, Stalin reacted to US pressure to give up Lwow by now offering Poland a border along the Western Neisse river and not just the East. They also threw in the 2000 year old German city of Stettin, now Szczecin, even though it was technically West of the Oder-Neisse line that was set up. Though Winston Churchill was hesitant in handing such a majority German and historically important city over to Poland only for its population to be expelled, the Western Powers were ultimately satisfied with this deal and finalized these borders at the Potsdam Conference. In establishing these borders, Stalin had created a strong Polish state at the expense of Germany and placated the Western Allies, successfully increasing their buffer between the Soviet Union and its new Capitalist rivals. Unfortunately, as a result of this arrangement, millions of Germans were expelled from their ancestral homes and ethnically cleansed by the Polish and Soviet governments. Thousands would die but it was the blood price for the millions of Russians and Poles who had suffered similar fates at the hands of the Germans only a few years earlier.