Why did the Allied powers move Poland’s western border after World War II?

The Allied powers moved Poland's western border after World War II to compensate Poland for territorial losses to the Soviet Union. At the Yalta Conference, the Allied leaders agreed to shift Poland's eastern boundary westward to approximately the Curzon Line. To compensate for this loss, they also agreed to move Poland's western frontier farther west, at the expense of Germany.1

While the exact location of this new border was debated, the Soviets advocated for the Oder-Neisse Line. By the time of the Potsdam Conference, the Soviet Red Army occupied the lands east of this proposed line, and the administration was transferred to a Polish provisional government. Despite protests from the United States and Great Britain, they accepted this arrangement, placing the territory east of the Oder-Neisse Line under Polish administrative control, with the exception of northern East Prussia, which was incorporated into the Soviet Union. The Potsdam conferees also permitted the deportation of German inhabitants from this area to Germany.1