World War II was a trial by fire for Poland. While in the 19th century Poles resisted efforts to denationalize them, during World War II physical extermination threatened the country's inhabitants. When the six years of brutal occupation were over in May 1945, 6,028,000 citizens, Gentiles and Jews, had perished, an estimated 22% of the prewar population. Only 10% of the country's dead died in military action. The remainder perished in executions, during deportations, as a result of the general deterioration of wartime existence, or in concentration or death camps. Nazi racial policies considered Jews, Slavs, and Gypsies “subhumans,” and Polish Jewry was almost entirely extinguished by the application of modern production line technology to death. Jews were killed only because they were Jews, but Nazi terror stalked every citizen. The situation was also perilous for those in Soviet-occupied eastern Poland between 1939 and 1941. As many as 1,500,000 inhabitants perished in executions, during massive deportations to Siberia, and in the brutal slave labor camps of the eastern Soviet Union.

Poles resisted. The political and military leaders organized a Government- in-Exile, which moved to London in 1940; an army-in-exile; and a civilian and military underground resistance in the country. Poland was a member of the Allied coalition, a signatory of the Atlantic Charter, and declared eligible for Lend Lease aid. Until Hitler turned on his Soviet ally, Poland's position within the Allied camp was clear. She was the innocent victim of aggression for whom Britain and France had declared war against Germany. Poland expected to regain her independence when the war ended, recover her prewar territories, and be compensated for German aggression.

Poland's political position changed dramatically in June 1941, when German forces invaded the Soviet Union. Geopolitical necessity made Stalin a welcome ally in the struggle against Nazi Germany. As the war progressed, the likelihood of the USSR having a deciding voice in the fate of postwar Eastern Europe increased. In the Spring of 1943, the USSR broke relations with the Polish Government-in-Exile over the question of responsibility for the massacre of Polish officers and non-commissioned officers at the Katyń Forest in the Spring of 1940.