The new border between postwar Poland and the Soviet Union along the Curzon Line ( requested by Premier Josef Stalin at the Yalta Conference with his western Allies ) had been ratified . There was an ensuing population exchange that affected close to half a million ethnic Ukrainians as well as about 1.1 million Poles and Polish Jews . While the east-central territories of the Soviet republics remained unchanged , the westernmost regions of Ukrainian and Belarusian SSR underwent dramatic expansion at the expense of the Second Polish Republic . The so-called repatriation pertained to rural populations as much as the inhabitants of provincial capitals stripped of their prewar economic catchment areas ( Grodno , Brest , Lviv , Przemyśl ) . About 480,000 people from Zakerzonia ( west of the Curzon Line ) , were moved eastward to the territory which became part of Soviet Ukraine and Belarus
Yalta Conference
Yalta Conference
Population exchange between Poland and Soviet Ukraine