Oder–Neisse line in the context of "Fall of communism in Poland"

The entirety of Germany was occupied and administered by the Allies of World War II, from the Berlin Declaration on 5 June 1945 to the establishment of West Germany on 23 May 1949. Unlike occupied Japan, Nazi Germany was stripped of its sovereignty and its government was entirely dissolved. After Germany formally surrendered on Tuesday, 8 May 1945, the four countries representing the Allies (the United States, United Kingdom, Soviet Union, and France) asserted joint authority and sovereignty through the Allied Control Council (ACC).

Germany after the war was a devastated country – roughly 80 percent of its infrastructure was in need of repair or reconstruction – which helped the idea that Germany was entering a new phase of history ("zero hour"). At first, Allied-occupied Germany was defined as all territories of Germany before the 1938 Nazi annexation of Austria. The Potsdam Agreement on 2 August 1945 defined the new eastern German border by giving Poland and the Soviet Union all regions of Germany east of the Oder–Neisse line (eastern parts of Pomerania, Neumark, Posen-West Prussia, East-Prussia and most of Silesia) and divided the remaining "Germany as a whole" into four occupation zones, each administered by one of the Allies.

East Germany, officially the German Democratic Republic (GDR), was a country in Central Europe from its formation on 7 October 1949 until its reunification with West Germany (Federal Republic of Germany) on 3 October 1990. Until 1989, it was generally viewed as a communist state and described itself as a socialist workers' and peasants' state. The economy of the country was centrally planned and state-owned. Although the GDR had to pay substantial war reparations to the Soviet Union, its economy became the most successful in the Eastern Bloc.

Before its establishment, the country's territory was administered and occupied by Soviet forces following the Berlin Declaration abolishing German sovereignty in World War II. The Potsdam Agreement established the Soviet-occupied zone, bounded on the east by the Oder–Neiße line. The GDR was dominated by the Socialist Unity Party of Germany (SED), a communist party, before being democratized and liberalized amid the revolutions of 1989; this paved the way for East Germany's reunification with West Germany. Unlike the government of West Germany, the SED did not see its state as the successor to the German Reich (1871–1945). In 1974, it abolished the goal of unification in the constitution. The SED-ruled GDR was often described as a Soviet satellite state; historians describe it as an authoritarian regime.

The Soviet invasion of Poland was a military conflict by the Soviet Union without a formal declaration of war. On 17 September 1939, the Soviet Union invaded Poland from the east, 16 days after Nazi Germany invaded Poland from the west. Subsequent military operations lasted for the following 20 days and ended on 6 October 1939 with the two-way division and annexation of the entire territory of the Second Polish Republic by Nazi Germany and the Soviet Union. This division is sometimes called the Fourth Partition of Poland. The Soviet (as well as German) invasion of Poland was indirectly indicated in the "secret protocol" of the Molotov–Ribbentrop Pact signed on 23 August 1939, which divided Poland into "spheres of influence" of the two powers. German and Soviet cooperation in the invasion of Poland has been described as co-belligerence.

The Red Army, which vastly outnumbered the Polish defenders, achieved its targets, encountering only limited resistance. Some 320,000 Poles were made prisoners of war. The campaign of mass persecution in the newly acquired areas began immediately. In November 1939 the Soviet government annexed the entire Polish territory under its control. Some 13.5 million Polish citizens who fell under the military occupation were made Soviet subjects following show elections conducted by the NKVD secret police in an atmosphere of terror, the results of which were used to legitimise the use of force. A Soviet campaign of political murders and other forms of repression, targeting Polish figures of authority such as military officers, police, and priests, began with a wave of arrests and summary executions. The Soviet NKVD sent hundreds of thousands of people from eastern Poland to Siberia and other remote parts of the Soviet Union in four major waves of deportation between 1939 and 1941.Soviet forces occupied eastern Poland until the summer of 1941 when Germany terminated its earlier pact with the Soviet Union and invaded the Soviet Union under the code name Operation Barbarossa. The area was under German occupation until the Red Army reconquered it in the summer of 1944. An agreement at the Yalta Conference permitted the Soviet Union to annex territories close to the Curzon Line (which almost coincided with all of their Molotov–Ribbentrop Pact portion of the Second Polish Republic), compensating the Polish People's Republic with the greater southern part of East Prussia and territories east of the Oder–Neisse line. The Soviet Union appended the annexed territories to the Ukrainian, Byelorussian and Lithuanian Soviet Socialist Republics.

The Frontier March of Posen–West Prussia (German: Grenzmark Posen-Westpreußen; Polish: Marchia Graniczna Poznańsko-Zachodniopruska) was a province of Prussia that existed from 1920 (de facto) and 1922 (de jure) until its dissolution in 1938. It comprised most of the former territory of West Prussia and parts of the Province of Posen that had remained within Germany following the territorial losses to the Second Polish Republic after the Greater Poland Uprising, and confirmed by the Treaty of Versailles. The province was created in 1922 as part of the Free State of Prussia within Weimar Germany, incorporating three remaining non-contiguous territories of the former provinces of Posen and West Prussia.

Schneidemühl (present-day Piła) served as the provincial capital. From 1934 onwards, the province came under the de facto administration of the Province of Brandenburg, and on 1 October 1938, the territory was officially dissolved by Nazi Germany. Its lands were subsequently incorporated into the neighbouring provinces of Pomerania, Brandenburg and Silesia. Following the end of World War II and the redrawing of borders, the entire area of the former province was incorporated into Poland, lying east of the Oder–Neisse line.

The Germany–Poland border (German: Grenze zwischen Deutschland und Polen, Polish: Granica polsko-niemiecka) is the state border between Poland and Germany, mostly along the Oder–Neisse line, with a total length of 467 km (290 mi). It stretches from the Baltic Sea in the north to the Czech Republic in the south.

The Oder (/ˈoʊdər/ OH-dər; Czech and Polish: Odra) is a river in Central Europe. It is Poland's second-longest river and third-longest within its borders after the Vistula and its largest tributary the Warta. The Oder rises in the Czech Republic and flows 742 kilometres (461 mi) through western Poland, later forming 187 kilometres (116 mi) of the border between Poland and Germany as part of the Oder–Neisse line. The river ultimately flows into the Szczecin Lagoon north of Szczecin and then into three branches (the Dziwna, Świna and Peene) that empty into the Bay of Pomerania of the Baltic Sea.