The Polish Underground State which functioned under the German occupation in the Second World War was an unique phenomenon in the whole history of European resistance movements.

In 1939, the territory of Poland was occupied and carved up between Nazi Germany and Soviet Russia. Then, the outbreak of the German-Russian war in 1941, meant that the entire Polish territory was overrun by the Germans, whose long-term aim was to exterminate the Poles. Occupied Warsaw, the Polish capital, remained for Poles the centre of their underground political life. Here in September 1939 was formed the Polish Victory Service (SZP), an underground political-military organisation which was the embryo of the subsequently developed underground state.

The Polish Underground State grew, and reached its most mature form in 1943–44. The highest authorities in this state were held by the Government Delegate at Home, in 1944 this was the deputy premier (of the acting government, in exile in London). The Government Delegates were, successively, Cyryl Ratajski (pseudonym 'Wartski'). Jan Piekalkiewicz (ps. "Julianski'), Jan Stanislaw Jankowski (ps. 'Soból'), Stefan Korbonski (ps. 'Zielinski').

The underground parliament was a representation of the most important political parties and groupings (the Political Consultative Committee - the Political Representation at Home) which in the period 1944–45 took the name of the Council of National Unity (RJN). In the underground parliament the more important political parties were represented: the Peasants' Party (SL), the Polish Socialist Party (PPS), the National Party (SN) and smaller groupings. The Polish Workers' Party (PPR) remained outside this parliamentary set up, refusing to recognise the constitutional Government of Poland and was not only wholly dependent on Russia, but also represented that state's interests.

In 1944, at the side of the Government Delegate's Office was the Home Council of Ministers (KRM). The Government Delegate directed the work of the Government Delegate's Office at Home which comprised 18 departments which corresponded to the government-in-exile's ministries in London. The Delegates' activities encompassed, to the extent that conditions of enemy occupation and terror allowed, all areas of organised society. First and foremost, this was secret education provided at every level - primary, secondary and higher. Secret courses were organised at the underground universities of Warsaw, Kraków (Cracow), Wilno (Vilna) and Lwów (Lvov). A very important influence on public opinion was also exerted through propaganda disseminated by the underground press. In general, throughout the entire wartime period, about 1500 different titles appeared. In addition to this, an underground administrative apparatus capable of undertaking work at the moment of restoration of an independent Polish state was organised.

The most important, however, was the Home Army (AK), the backbone in the struggle with the occupant. This armed force, evolved from SZP, through the Union of Armed Combat (ZWZ), to reach its apogee as the AK. This constituted an integral part of the Polish Armed Forces (PSZ) and remained under the overall command of the Polish Commander-in-Chief who remained abroad. The successive commanders in the SZP-ZWZ-AK chain of development were General Michal Tokarzewski-Karaszewicz (pseudonym 'Torwid'), General Stefan Rowecki (ps. 'Grot'), General Tadeusz Komorowski (ps. 'Bór'), and General Leopold Okulicki ('Niedzwiadek").

At the top of the command structure was the Home Army High Command (KG AK) which consisted of seven staff bureaus and miscellaneous specialist units and detachments. The territory of Poland, in its interwar shape, was divided into areas and regions, each of which had its own scaled down version of the KG AK.

The chief task of the AK was to prepare and execute a general uprising in Poland coordinated with the Allies in the final phase of the war, which would liberate Poland from the occupant in one fell swoop. The on-going struggle concentrated on self-defence (freeing prisoners and hostages, defence against pacification measures), and striking at the occupant's apparatus of terror (the physical liquidation of Gestapo and SS functionaries). These activities were conducted by a special combat department known as the Diversionary Directorate (Kierownictwo Dywersji) - Kedyw for short, under the command of Colonel August Emil Fieldorf. Besides this special partisan detachments were created, designed to train soldiers for the coming uprising through small scale on-going skirmishing.

As a result of the unification of the various underground military organisations, the AK finally emerged in 1944 as an umbrella organisation for a national military force of over 350,000 soldiers. The intelligence work of the AK in a wide area of Europe was of capital importance to Allied strategists. Among other achievements, the production centre of the V1 rocket was pinpointed and essential V2 parts were won and passed on to London. Psychological warfare was also waged, in which Action 'N' was mounted to create the illusion of an internal German opposition movement to Hitler.

The AK single-handedly liberated a number of places from German control. The Warsaw Uprising, due to a lack of Allied support, collapsed after a 63 day battle. In the face of the resumed Soviet offensive of 19 January 1945, the AK was dissolved. The civil authorities of the Polish Underground State followed suit in July 1945. AK soldiers suffered repression from the Russians either through the Moscow-controlled Polish communists, or directly at the hands of the NKVD [ nkvd ] (the Soviet security police service).