The Western powers remained unaware of the top secret Nazi-Soviet Pact in 1939 , which paved the way for World War II . The German surrender in May 1945 was followed by a massive change in the political geography of Europe . Poland 's borders were redrawn by the Allies according to the demands made by Josef Stalin during the Tehran Conference , confirmed as not negotiable at the Yalta Conference of 1945 . The Polish government-in-exile was excluded from the negotiations . The territory of Poland was reduced by approximately 20 percent . Before the end of 1946 some 1.8 million Polish citizens were expelled and forcibly resettled within the new borders . For the first time in its history Poland became a homogeneous one nation-state by force , with the national wealth reduced by 38 percent . Poland 's financial system had been destroyed . Intelligentsia was largely obliterated along with the Jews , and the population reduced by about 33 percent
Yalta Conference
Yalta Conference
The Holocaust in Poland