The invasion of Poland, also known as the September Campaign, Polish Campaign, and Polish Defensive War of 1939, was a joint attack on the Republic of Poland by Nazi Germany, the Slovak Republic, and the Soviet Union, which marked the beginning of World War II. The invasion began on 1 September 1939 when Germany marched into Poland, preceded by the Danzig crisis and by the Molotov–Ribbentrop Pact between Germany and the Soviet Union, which was signed one week before and approved by the Supreme Soviet of the Soviet Union one day before the invasion. On 17 September the Soviet Union invaded as well, acting on a secret protocol of the pact, which assigned parts of Poland to them. The campaign ended on 6 October with Germany and the Soviet Union dividing the whole of Poland under the terms of the German–Soviet Frontier Treaty.