In 1945 , the territories east of the Oder-Neisse line ( Silesia , most of Pomerania , East Brandenburg and East Prussia ) were occupied by Soviet and Soviet-controlled Polish military forces . Polish militia and military started expulsions before the Potsdam Conference , referred to as " wild expulsions " ( ) , affecting between 700,000 and 800,000 Germans . The Polish communists ordered the expulsion of Germans : " We must expel all the Germans because countries are built on national lines and not on multi-national ones " was demanded by participants of a Plenum of the Central Committee of the Polish Workers Party on May 20 – 21 , 1945 . On the same Plenum , the head of the Central Committee , Władysław Gomułka , ordered : " There has to be a border patrol at the border Oder-Neisse line and the Germans have to be driven out . The main objective has to be the cleansing of the terrain of Germans , the building of a nation state " . To ensure the Oder – Neisse line would be accepted as the new Polish border at a future Allied Conference ( Potsdam Conference ) , up to 300,000 Germans living close to the rivers ' eastern bank were expelled subsequently . On May 26 , 1945 , the Central Committee ordered all Germans to be expelled within one year and the area settled with some 3.5 million ethnic Poles ; 2.5 million of them were already re-settled by summer
Potsdam Conference
Potsdam Conference
Flight and expulsion of Germans from Poland during and after World War II