The territorial changes of Poland immediately after World War II were very extensive, the Oder-Neisse line became Poland's western border and the Curzon Line its eastern border. In 1945, after the defeat of Nazi Germany, Poland's borders were redrawn in accordance with the decisions made first by the Allies at the Tehran Conference of 1943 where the Soviet Union demanded the recognition of the line proposed by British Foreign Secretary Lord Curzon in 1920.
The same Soviet stance was repeated by Joseph Stalin again at the Yalta Conference with Roosevelt and Churchill in February 1945, but a lot more forcefully in the face of the looming German defeat. The new borders were ratified at the Potsdam Conference of August 1945 exactly as proposed by Stalin who already controlled the whole of East-Central Europe.Harry Truman remembered:
I remember at Potsdam, we got to discussing a matter in eastern Poland, and it was remarked by the Prime Minister of Great Britain that the Pope would not be happy over the arrangement of that Catholic end of Poland. And the Generalissimo, the Prime Minister of Russia leaned on the table, and he pulled his mustache like that, and looked over to Mr. Churchill and said: Mr. Churchill, Mr. Prime Minister, how many divisions did you say the Pope had?
Large territories of Poland were annexed by the Soviet Union and today those areas form part of Lithuania, Belarus and Ukraine. Poland was instead given the Free State of Danzig and the German areas east of the rivers Oder and Neisse (see Recovered Territories). The ethnic cleansing of both Polish and Germans 1945-46 included many millions of people. The Polish territory in 1919-39 covered an area of 386,418 square kilometres (149,197 square miles). But from 1947, Poland's territory was reduced to 312,679 square kilometres (120,726 square miles), so the country lost 73,739 square kilometres (28,471 square miles) of land. This difference amounts almost to the size of the Czech Republic, although Poland ended up with a much longer coastline on the Baltic Sea compared to its 1939 borders. In addition, the infrastructure in the former eastern territories of Germany was more developed than in the territories annexed by the Soviet Union.
The prewar eastern Polish territories of Kresy, which the Red Army had overrun during the Nazi-Soviet invasion of Poland in 1939 (excluding the Bia?ystok region) were permanently annexed by the USSR, and most of their Polish inhabitants expelled. As a result of the Potsdam agreement to which Poland's government-in-exile was not invited, Poland lost 179,000 square kilometres (69,000 square miles) (45%) of prewar territories in the east, including over 12 million citizens of whom 4,3 million were ethnically the speakers of Polish. Today, these territories are part of sovereign Belarus, Ukraine, and Lithuania.
In turn, postwar Poland was assigned considerably smaller territories to the west including the prewar Free City of Danzig and the former territory of Germany east of the Oder-Neisse line, consisting of the southern portion of East Prussia and most of Pomerania, Neumark (East Brandenburg), and German Silesia. The German population fled or was forcibly expelled before these Recovered Territories (official term) were repopulated with Poles expelled from the eastern regions and those from central Poland. The small area of Zaolzie, which had been annexed by Poland in late 1938, was returned to Czechoslovakia on Stalin's orders.