The Home Army , the largest underground force in Europe , and other resistance organizations in occupied Poland provided intelligence that enabled successful operations later in the war and led to uncovering of Nazi war crimes ( i.e. , death camps ) to the Western Allies . Notable Polish units fought in every campaign in North Africa and Europe ( outside the Balkans ) . The Soviet Union recognized the London-based government at first . But it broke diplomatic relations after the Katyn massacre of Polish nationals was revealed . In 1943 , the Soviet Union organized the Polish People 's Army under Zygmunt Berling , around which it constructed the post-war successor state People 's Republic of Poland
Katyn massacre
Katyn massacre
Allies of World War II