 Complete the following phrase:  The Invasion of Poland 1 September  6 October 1939, also known as the September campaign , 1939 defensive war  and Poland campaign , was an attack on the Republic of Poland by Nazi Germany and the Soviet Union which marked the beginning of World War II. The German invasion began on 1 September 1939, one week after the signing of the MolotovRibbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact. The 