The Invasion of Poland, also known as the September Campaign or 1939 Defensive War (Polish: Kampania wrześniowa or Wojna obronna 1939 roku) in Poland and the Poland Campaign (German: Polenfeldzug) or Fall Weiss (Case White) in Germany, was an invasion of Poland by Germany, the Soviet Union, and a small Slovak contingent that marked the beginning of World War II in Europe. The German invasion began on 1 September 1939, one week after the signing of the Molotov–Ribbentrop Pact, while the Soviet invasion commenced on 17 September 1939 following the Molotov-Tōgō agreement which terminated the Nomonhan incident on 16 September 1939. The campaign ended on 6 October 1939 with Germany and the Soviet Union dividing and annexing the whole of Poland.

Source: Wikipedia