Western betrayal in the context of "Invaded Poland"

The invasion of Poland, also known as the September Campaign, Polish Campaign, and Polish Defensive War of 1939 (1 September – 6 October 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 aim of the invasion was to disestablish Poland as a sovereign country, with its citizens destined for extermination. German and Slovak forces invaded Poland from the north, south, and west the morning after the Gleiwitz incident. As the Wehrmacht advanced, Polish forces withdrew from their forward bases of operation close to the Germany–Poland border to more established defense lines to the east. After the mid-September Polish defeat in the Battle of the Bzura, the Germans gained an undisputed advantage. Polish forces then withdrew to the southeast where they prepared for a long defence of the Romanian Bridgehead and awaited expected support and relief from France and the United Kingdom. On 3 September, based on their alliance agreements with Poland, the United Kingdom and France declared war on Germany; in the end their aid to Poland was very limited. France invaded a small part of Germany in the Saar Offensive, and the Polish army was effectively defeated even before the British Expeditionary Force could be transported to continental Europe.

The Polish Underground State (Polish: Polskie Państwo Podziemne, also known as the Polish Secret State) 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 (and its institutions) 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, but also civilian structures, such as justice, education, culture and social services.

Although the Underground State enjoyed broad support throughout much of the war, it was not supported or recognized by the communists and some of the right-wing extremists. The influence of the communists eventually declined amid military reversals (most notably, the failure of the Warsaw Uprising) and the growing hostility of the USSR. The Soviet Union had created an alternative, puppet government in 1944 (the Polish Committee of National Liberation) and ensured it formed the basis of the post-war government in Poland. During the Soviet-backed communist takeover of Poland at the end of the war, many Underground State members were prosecuted as alleged traitors and died in captivity. Abandoned by the Western Allies, finding it impossible to negotiate with the Soviets, and wishing to avoid a civil war, the key institutions of the Underground State dissolved themselves in the first half of 1945.