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 outlined in the genocidal ''Generalplan Ost''.