The Invasion of Poland, also known as the September Campaign or 1939 Defensive War in Poland and the Poland Campaign or Fall Weiß 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 following the Molotov-Tōgō agreement which terminated the Russian and Japanese hostilities in the east on 16 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland.
The morning after the Gleiwitz incident, German forces invaded Poland from the north, south, and west. As the Germans advanced, Polish forces withdrew from their forward bases of operation close to the Polish–German border to more established lines of defence to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage.