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 : Poland lost large territories to the Soviet Union ( today those areas are located in Lithuania , Belarus and Ukraine ) . Poland was instead given the Free State of Danzig and the German areas east of the rivers Oder and Neisse . 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 . But from 1947 , Poland 's territory was reduced to , so the country lost 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
Potsdam Conference
Potsdam Conference
Territorial changes of Poland immediately after World War II