Soviet invasion of Poland in the context of "Lavrentiy Beria"

Belarus, officially the Republic of Belarus, is a landlocked country in Eastern Europe. It is bordered by Russia to the east and northeast, Ukraine to the south, Poland to the west, and Lithuania and Latvia to the northwest. Belarus spans an area of 207,600 square kilometres (80,200 sq mi) with a population of 9.1 million. The country has a hemiboreal climate and is administratively divided into six regions. Minsk is the capital and largest city; it is administered separately as a city with special status.

For most of the medieval period, the lands of modern-day Belarus was ruled by independent city-states such as the Principality of Polotsk. Around 1300 these lands came fully under the Grand Duchy of Lithuania and subsequently by the Polish–Lithuanian Commonwealth; this period lasted for 500 years until the 1792-1795 partitions of Poland-Lithuania placed Belarus within the Russian Empire for the first time. In the aftermath of the Russian Revolution in 1917, different states arose competing for legitimacy amid the Civil War, ultimately ending in the rise of the Byelorussian SSR, which became a founding constituent republic of the Soviet Union in 1922. After the Polish-Soviet War (1918–1921), Belarus lost almost half of its territory to Poland. Much of the borders of Belarus took their modern shape in 1939, when some lands of the Second Polish Republic were reintegrated into it after the Soviet invasion of Poland, and were finalized after World War II. During World War II, military operations devastated Belarus, which lost about a quarter of its population and half of its economic resources. In 1945, the Byelorussian SSR became a founding member of the United Nations, along with the Soviet Union. The republic was home to a widespread and diverse anti-Nazi insurgent movement which dominated politics until well into the 1970s, overseeing Belarus's transformation from an agrarian to an industrial economy.

The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during World War II (1939–1945) to oppose the Axis powers. Its principal members were the "Big Four" – the United Kingdom, Soviet Union, United States, and China.

Membership in the Allies varied during the course of the war. When the conflict broke out on 1 September 1939, the Allied coalition consisted of the United Kingdom, France, and Poland, as well as their respective dependencies, such as British India. They were joined by the independent dominions of the British Commonwealth: Canada, Australia, New Zealand and South Africa. Consequently, the initial alliance resembled that of the First World War. As Axis forces began invading northern Europe and the Balkans, the Allies added the Netherlands, Belgium, Norway, Greece, and Yugoslavia. The Soviet Union, which initially had a nonaggression pact with Germany and participated in its invasion of Poland, joined the Allies after the German invasion of the Soviet Union in June 1941. The United States, while providing some material support to European Allies since September 1940, remained formally neutral until the Japanese attack on Pearl Harbor in December 1941, after which it declared war and officially joined the Allies. China had already been at war with Japan since 1937, and formally joined the Allies in December 1941.

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 European theatre of World War II was one of the two main theatres of combat during World War II, taking place from September 1939 to May 1945. The Allied powers (including the United Kingdom, the United States, the Soviet Union and France) fought the Axis powers (including Nazi Germany and the Kingdom of Italy) on both sides of the continent in the Western and Eastern fronts. There was also conflict in the Scandinavian, Mediterranean and Balkan regions. It was an intense conflict that led to at least 39 million deaths and a dramatic change in the balance of power in the continent.

During the 1930s, Adolf Hitler, the leader of Nazi Germany, expanded German territory by annexing all of Austria and the Sudetenland region of Czechoslovakia in 1938. This was motivated in part by Germany's racial policy that believed the country needed to expand for the pseudoscientific "Aryan race" to survive. They were aided by Italy, another fascist state which was led by Benito Mussolini. World War II started with Germany's invasion of Poland on 1 September 1939, and the Soviet Union, led by Joseph Stalin, joined the invasion later that month. The two nations then partitioned Poland between them.

Western Belorussia or Western Belarus (Belarusian: Заходняя Беларусь, romanized: Zachodniaja Biełaruś; Polish: Zachodnia Białoruś; Russian: Западная Белоруссия, romanized: Zapadnaya Belorussiya) is a historical region of modern-day Belarus which belonged to the Second Polish Republic during the interwar period. For twenty years before the 1939 invasion of Poland, it was the northern part of the Polish Kresy macroregion. Following the end of World War II in Europe, most of Western Belorussia was ceded to the Soviet Union by the Allies, while some of it, including Białystok, was given to the Polish People's Republic. Until the dissolution of the Soviet Union in 1991, Western Belorussia formed the western part of the Byelorussian Soviet Socialist Republic (BSSR). Today, it constitutes the west of modern Belarus.

Created by the USSR after the conquest of Poland, the new western provinces of Byelorussian SSR acquired from Poland included Baranavichy, Belastok, Brest, Vileyka and the Pinsk Regions. The majority of Belastok Region was returned to Poland and the rest of the regions were reorganized one more time after the Soviet liberation of Belarus into the contemporary western provinces of Belarus which include all of Grodno and Brest regions, as well as parts of today's Minsk and Vitebsk regions. Vilnius was returned by the USSR to the Republic of Lithuania which soon after that became the Lithuanian SSR.

The Second Polish Republic, officially known at the time as the Republic of Poland, was a country in Central and Eastern Europe that existed between 7 October 1918 and 6 October 1939 after being established in the final stage of World War I. The Second Republic was taken over in 1939, after it was invaded by Nazi Germany, the Soviet Union, and the Slovak Republic, marking the beginning of the European theatre of the Second World War. The Polish government-in-exile was established in Paris and later London after the fall of France in 1940.

When, after several regional conflicts, most importantly the victorious Polish-Soviet war, the borders of the state were finalized in 1922, Poland's neighbours were Czechoslovakia, Germany, the Free City of Danzig, Lithuania, Latvia, Romania, and the Soviet Union. It had access to the Baltic Sea via a short strip of coastline known as the Polish Corridor on either side of the city of Gdynia. Between March and August 1939, Poland also shared a border with the then-Hungarian governorate of Subcarpathia. In 1938, the Second Republic was the sixth largest country in Europe. According to the 1921 census, the number of inhabitants was 25.7 million. By 1939, just before the outbreak of World War II, this had grown to an estimated 35.1 million. Almost a third of the population came from minority groups: 13.9% Ukrainians; 10% Ashkenazi Jews; 3.1% Belarusians; 2.3% Germans and 3.4% Czechs and Lithuanians. At the same time, a significant number of ethnic Poles lived outside the country's borders.