However , most of the German inhabitants , which then consisted primarily of women , children and old men , did manage to escape the Red Army as part of the largest exodus of people in human history : " A population which had stood at 2.2 million in 1940 was reduced to 193,000 at the end of May 1945 . " Following Nazi Germany 's defeat in World War II in 1945 , East Prussia was partitioned between Poland and the Soviet Union according to the Potsdam Conference , pending a final peace conference with Germany . Since a peace conference never took place , the region was effectively ceded by Germany . . Southern East Prussia was placed under Polish administration , while northern East Prussia was divided between the Soviet republics of Russia ( the Kaliningrad Oblast ) and Lithuania ( the constituent counties of the Klaipėda Region ) . The city of Königsberg was renamed Kaliningrad in 1946 . Most of the German population of the province had left during the evacuation at the end of the war , but several hundreds of thousands died during the years 1944 – 46 and the remainder were subsequently expelled
Potsdam Conference
Potsdam Conference
East Prussia