The final decision to move Poland 's boundary westward was made by the United States , the United Kingdom and the Soviet Union at the Yalta Conference in February 1945 , shortly before the end of the war . The precise location of the border was left open ; the western Allies also accepted in general the principle of the Oder River as the future western border of Poland and of population transfer as the way to prevent future border disputes . The open question was whether the border should follow the Eastern or Lusatian Neisse rivers , and whether Stettin , the traditional seaport of Berlin , should remain German or be included in Poland
Yalta Conference
Yalta Conference
Former eastern territories of Germany