Territories of Poland annexed by the Soviet Union in the context of "Soviet invasion of Poland"

During World War II, Poland was occupied by Nazi Germany and the Soviet Union (and Slovakia) following the invasion in September 1939, and it was formally concluded with the defeat of Germany by the Allies in May 1945. Throughout the entire course of the occupation, the territory of Poland was divided between Nazi Germany and the Soviet Union (USSR), both of which intended to eradicate Poland's culture and subjugate its people. In the summer-autumn of 1941, the lands which were annexed by the Soviets were overrun by Germany in the course of the initially successful German attack on the USSR. After a few years of fighting, the Red Army drove the German forces out of the USSR and crossed into Poland from the rest of Central and Eastern Europe.

Sociologist Tadeusz Piotrowski argues that both occupying powers were hostile to the existence of Poland's sovereignty, people, and the culture and aimed to destroy them. Before Operation Barbarossa, Germany and the Soviet Union coordinated their Poland-related policies, most visibly in the four Gestapo–NKVD conferences, where the occupiers discussed their plans to deal with the Polish resistance movement.

The General Government (German: Generalgouvernement, IPA: [ɡenəˈʁaːlɡuvɛʁnəˌmã] ; Polish: Generalne Gubernatorstwo; Ukrainian: Генеральна губернія, romanized: Heneralna huberniia), formally the General Governorate for the Occupied Polish Region (German: Generalgouvernement für die besetzten polnischen Gebiete), was a German zone of occupation established after the invasion of Poland by Nazi Germany, Slovakia and the Soviet Union in 1939 at the onset of World War II. The newly occupied Second Polish Republic was split into three zones: the General Government in its centre, Polish areas annexed by Nazi Germany in the west, and Polish areas annexed by the Soviet Union in the east. The territory was expanded substantially in 1941, after the German Invasion of the Soviet Union, to include the new District of Galicia. The area of the Generalgouvernement roughly corresponded with the Austrian part of the Polish–Lithuanian Commonwealth after the Third Partition of Poland in 1795.

The basis for the formation of the General Government was the "Annexation Decree on the Administration of the Occupied Polish Territories". Announced by Hitler on October 8, 1939, it claimed that the Polish government had totally collapsed. This rationale was utilized by the German Supreme Court to reassign the identity of all Polish nationals as stateless subjects, with the exception of the ethnic Germans of interwar Poland—who, disregarding international law, were named the only rightful citizens of Nazi Germany.

During World War II, the Soviet Union occupied and annexed several countries allocated to it in the secret Molotov–Ribbentrop Pact of 1939. These included the eastern regions of Poland (incorporated into three different SSRs), as well as Latvia (became Latvian SSR), Estonia (became Estonian SSR), Lithuania (became Lithuanian SSR), part of eastern Finland (became Karelo-Finnish SSR) and eastern Romania (became the Moldavian SSR and part of Ukrainian SSR). Apart from the Molotov–Ribbentrop Pact and post-war division of Germany, the Soviets also occupied and annexed Carpathian Ruthenia from Czechoslovakia in 1945 (became part of Ukrainian SSR). These occupations lasted until the dissolution of the Soviet Union in 1990 and 1991.

Below is a list of various forms of military occupations by the Soviet Union resulting from both the Soviet pact with Nazi Germany (ahead of World War II), and the ensuing Cold War in the aftermath of Allied victory over Germany.

The Polish Committee of National Liberation (Polish: Polski Komitet Wyzwolenia Narodowego, PKWN), also known as the Lublin Committee, was an executive governing authority established by the Soviet-backed communists in Poland at the later stage of World War II. It was officially proclaimed on 22 July 1944 in Chełm, installed on 26 July in Lublin and placed formally under the direction of the State National Council (Krajowa Rada Narodowa, KRN). The PKWN was a provisional entity functioning in opposition to the London-based Polish government-in-exile, which was recognized by the Western allies. The PKWN exercised control over Polish territory retaken from Nazi Germany by the Soviet Red Army and the Polish People's Army. It was sponsored and controlled by the Soviet Union and dominated by Polish communists.

Soviet partisans were members of resistance movements that fought a guerrilla war against Axis forces during World War II in the Soviet Union, the previously Soviet-occupied territories of interwar Poland in 1941–45 and eastern Finland. The activity emerged after Nazi Germany's Operation Barbarossa was launched from mid-1941 on. It was coordinated and controlled by the Soviet government and modeled on that of the Red Army.

The partisans made a significant contribution to the war by countering German plans to exploit occupied Soviet territories economically, gave considerable help to the Red Army by conducting systematic attacks against Germany's rear communication network, disseminated political rhetoric among the local population by publishing newspapers and leaflets, and succeeded in creating and maintaining feelings of insecurity among Axis forces.

The Border Agreement between Poland and the USSR of 16 August 1945 established the borders between the Soviet Union (USSR) and the People's Republic of Poland. It was signed by the Provisional Government of National Unity (Tymczasowy Rząd Jedności Narodowej) formed by the Polish communists. According to the treaty, Poland officially accepted the ceding its pre-war Eastern territory to the USSR (Kresy) which was decided earlier in Yalta already. Some of the territory along the Curzon line, established by Stalin during the course of the war, was returned to Poland. The treaty also recognised the division of the former German East Prussia and ultimately approved the finalised delimitation line between the Soviet Union and Poland: from the Baltic Sea, to the border tripoint with Czechoslovakia in the Carpathians. The agreement entered into force on 5 February 1946.

In the course of Operation Barbarossa, Nazi Germany invaded Estonia in July–December 1941, and occupied the country until 1944. Estonia had gained independence in 1918 from the then-warring German and Russian Empires. However, in the wake of the August 1939 Nazi-Soviet Pact, the Soviet Union had invaded and occupied Estonia in June 1940, and the country was formally annexed into the USSR in August 1940.

In the summer of 1941, the German invaders were at first seen by most Estonians as liberators from Soviet terror, since the Germans arrived only a week after the mass deportation of tens of thousands of people from Estonia and other territories occupied by the USSR in 1939–1941: eastern Poland, Latvia, Lithuania, Bessarabia and Northern Bukovina. Despite high hopes for Estonian independence, the people there soon realized that the Germans were just a different occupying power.