The invasion of Poland, also known as September campaign, 1939 defensive war and Poland campaign, was an attack on the Second Polish Republic 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 Molotov–Ribbentrop Pact between Germany and the Soviet Union, and one day after the Supreme Soviet of the Soviet Union had approved the pact.
The Soviets invaded Poland on 17 September. The campaign ended on 6 October with Germany and the Soviet Union dividing and annexing the whole of Poland under the terms of the German–Soviet Frontier Treaty.
On 8 October, after an initial period of military administration, Germany directly annexed western Poland and the former Free City of Danzig and placed the remaining block of territory under the administration of the newly established General Government.
The Soviet Union incorporated its newly acquired areas into its constituent Byelorussian and Ukrainian republics, and immediately started a campaign of Sovietization.
In the aftermath of the invasion, a collective of underground resistance organizations formed the Polish Underground State within the territory of the former Polish state.
Many of the military exiles that managed to escape Poland subsequently joined the Polish Armed Forces in the West, an armed force loyal to the Polish government-in-exile.