Following the Tehran Conference ( November – December 1943 ) Joseph Stalin and Winston Churchill made it clear that the Soviets would keep the Polish territories east of the Curzon Line and offered Poland territorial compensation in the West . The final decision to move Poland 's boundary westward , preconditioning the expulsion of Germans , was made by Britain , the Soviet Union , and the United States at the Yalta Conference in February 1945 , when the Curzon line was irrevocably fixed as the future Polish-Soviet border . The precise location of the Polish western border was left open and , though basically the Allies had agreed on population transfers , the extent remained questioned . Concerning the post-war western frontier of Poland , the agreement simply read : " '' If a specific problem such as the frontiers of liberated Poland and the complexion of its government allowed no easy solution , hopes were held out for the future discussion of all outstanding problems in an amicable manner . '' " Upon gaining control of these lands , the Soviet and Polish-Communist authorities started to expel the German population
Yalta Conference
Yalta Conference
Flight and expulsion of Germans from Poland during and after World War II