Curzon Line in the context of "History of Poland (1945–1989)"

The history of Poland from 1945 to 1989 spans the period of Marxist–Leninist regime in Poland after the end of World War II. These years, while featuring general industrialization, urbanization and many improvements in the standard of living, were marred by early Stalinist repressions, social unrest, political strife and severe economic difficulties.Near the end of World War II, the advancing Soviet Red Army, along with the Polish Armed Forces in the East, pushed out the Nazi German forces from occupied Poland. In February 1945, the Yalta Conference sanctioned the formation of a provisional government of Poland from a compromise coalition, until postwar elections. Joseph Stalin, the leader of the Soviet Union, manipulated the implementation of that ruling. A practically communist-controlled Provisional Government of National Unity was formed in Warsaw by ignoring the Polish government-in-exile based in London since 1940.

During the subsequent Potsdam Conference in July–August 1945, the three major Allies ratified a massive westerly shift of Poland's borders and approved its new territory between the Oder–Neisse line and the Curzon Line. The area of Poland was reduced in comparison to its pre-World War II extent and geographically resembled that of the medieval early Piast dynasty era. Following the destruction of the Polish-Jewish population in the Holocaust, the flight and expulsion of Germans in the west, resettlement of Ukrainians in the east, and the expulsion and resettlement of Poles from the Eastern Borderlands (Kresy), Poland became for the first time in its history an ethnically homogeneous nation-state without prominent minorities. The new government solidified its political power, while the Polish United Workers' Party (PZPR) under Bolesław Bierut gained firm control over the country, which would remain an independent state within the Soviet sphere of influence. The July Constitution was promulgated on 22 July 1952 and the country officially became the Polish People's Republic (PRL).

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 Polish population transfers in 1944–1946 from the eastern half of prewar Poland (also known as the expulsions of Poles from the Kresy macroregion), were the forced migrations of Poles toward the end and in the aftermath of World War II. These were the result of a Soviet Union policy that had been ratified by the main Allies of World War II. Similarly, the Soviet Union had enforced policies between 1939 and 1941 which targeted and expelled ethnic Poles residing in the Soviet zone of occupation following the Nazi-Soviet invasion of Poland. The second wave of expulsions resulted from the retaking of Poland from the Wehrmacht by the Red Army. The USSR took over territory for its western republics.

The postwar population transfers were part of an official Soviet policy that affected more than one million Polish citizens, who were removed in stages from the Polish areas annexed by the Soviet Union. After the war, following Soviet demands laid out during the Tehran Conference of 1943, Kresy was formally incorporated into the Ukrainian, Belarusian and Lithuanian republics of the Soviet Union. This was agreed at the Potsdam Conference of Allies in 1945, to which the Polish government-in-exile was not invited.

The Treaty of Riga was signed in Riga, Latvia, on 18 March 1921 between Poland on one side and Soviet Russia (acting also on behalf of Soviet Belarus) and Soviet Ukraine on the other, ending the Polish–Soviet War (1919–1921). The chief negotiators of the peace were Jan Dąbski for the Polish side and Adolph Joffe for the Soviet side.

Under the treaty, Poland recognized Soviet Ukraine and Belarus, abrogating its 1920 Treaty of Warsaw with the Ukrainian People's Republic. The Treaty of Riga established a Polish–Soviet border about 250 kilometres (160 mi) east of the Curzon Line, incorporating large numbers of Ukrainians and Belarusians into the Second Polish Republic. Poland, which agreed to withdraw from areas further east (notably Minsk), renounced claims to the Polish–Lithuanian Commonwealth's border prior to the 1772 First Partition of Poland, recovering only those eastern regions (Kresy) lost to Russia in the 1795 Third Partition. Russia and Ukraine agreed to withdraw their claims to lands west of the demarcated border line. Poland, by recognising the puppet states of the USSR and simultaneously withdrawing recognition of the UPR (its only ally in the Polish-Bolshevik war), was in fact giving up on the federation programme, while Russia approved of the fact that the whole of Galicia, as well as the territories of the former Russian Empire, inhabited largely by non-Polish people, were to be found within Poland's borders. The treaty also addressed matters of sovereignty, citizenship, national minorities, repatriation, and diplomatic and commercial relations. The Treaty lasted until the invasion of Poland by the Soviet Union in 1939, and their borders were redefined by an agreement in 1945.

54°30′N 25°45′E / 54.500°N 25.750°E / 54.500; 25.750The Republic of Central Lithuania (Polish: Republika Litwy Środkowej, Lithuanian: Vidurio Lietuvos Respublika), commonly known as the Central Lithuania, and the Middle Lithuania (Polish: Litwa Środkowa, Lithuanian: Vidurinė Lietuva, Belarusian: Сярэдняя Літва, romanized: Siaredniaja Litva), was an unrecognized short-lived puppet state of Poland, that existed from 1920 to 1922. It was founded on 12 October 1920, after the successful Żeligowski Mutiny, during which the volunteer 1st Lithuanian–Belarusian Division under the command of general Lucjan Żeligowski seized the Vilnius Region that Lithuania made claims to. It was incorporated into Poland on 18 April 1922.

Vilnius, the historical capital of Lithuania, had a majority Polish-speaking population with a Lithuanian-speaking minority of only 2–3%. Therefore, the Polish authorities decided that the region should belong to the newly-established Polish state, and attempted to implement this idea using military force, ignoring the Curzon Line and taking advantage of the fact that victorious Poles after the Battle of Warsaw were advancing to the East against the Bolsheviks in the Polish–Soviet War. The Poles reasoned that they were justified in seizing as many mixed areas as possible, as well as to protect the Catholic, predominantly Polish-speaking population in disputed areas. Being utterly militarily outmatched, Lithuania did not stand a chance to maintain control of the region. This led to the renewal of Polish–Lithuanian War, where the so-called Żeligowski Mutiny, secretly ordered by Józef Piłsudski, was part of the military operation, fully supported and backed on the flanks by the Polish army, and consequently to the establishment of the so-called Republic of Central Lithuania.

At the end of World War II, Poland underwent major changes to the location of its international border. In 1945, after the defeat of Nazi Germany, the Oder–Neisse line became its western border, resulting in gaining the Recovered Territories from Germany. The Curzon Line became its eastern border, resulting in the loss of the Eastern Borderlands to the Soviet Union.