World War II began with the Nazi German invasion of Poland on 1 September 1939 , followed by the Soviet invasion of Poland on 17 September . On 28 September 1939 Warsaw fell . As agreed in the Molotov – Ribbentrop Pact , Poland was split into two zones , one occupied by Nazi Germany , the other by the Soviet Union . In 1939 – 41 , the Soviets deported hundreds of thousands of Poles . The Soviet NKVD executed thousands of Polish prisoners of war ( inter alia Katyn massacre ) ahead of the Operation Barbarossa . German planners had in November 1939 called for " the complete destruction " of all Poles and their fate , as well as many other Slavs , was outlined in genocidal '' Generalplan Ost ''
Katyn massacre
Katyn massacre
Poland