All prewar German territories east of the line and within the 1937 German boundaries – comprising nearly one-fourth ( 23.8 percent ) of the ( pre-Nazi ) Weimar Republic – were annexed under border changes promulgated at the postwar Potsdam Conference , with most becoming part of Poland . The small remainder , consisting of northern East Prussia with the German city of Königsberg ( now renamed Kaliningrad ) , was allocated to the Soviet Union as the Kaliningrad Oblast of the Russian SFSR ( today Russia ) . Nearly all of the German population in these territories – estimated at approximately 12 million as of autumn 1944 – fled or were later expelled
Potsdam Conference
Potsdam Conference
Oder–Neisse line