The Polish Underground State [a] was a single political and military entity formed by the union of resistance organizations in occupied Poland that were loyal to the Government of the Republic of Poland in exile in London. The first elements of the Underground State were established in the final days of the German and Soviet invasion of Poland, in late September 1939. The Underground State was perceived by supporters as a legal continuation of the pre-war Republic of Poland that waged an armed struggle against the country's occupying powers: Nazi Germany and the Soviet Union. The Underground State encompassed not only military resistance, one of the largest in the world,[b] but also civilian structures, such as justice, education, culture and social services.

The Invasion of Poland, also known as the September Campaign, Polish Campaign, War of Poland of 1939, 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 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. The invasion is also known in Poland as the September campaign or 1939 defensive war and known in Germany as the Poland campaign.