The territory forming today's western part of Belarus include in particular, Hrodna and Brest voblasts, as well as parts of today's Minsk and Vitsebsk oblasts. The historical population of West Belarus included Belarusians, Poles, Lithuanians, Jews, and Russians. Many peasants in Polesia (the Poleszuks) declared themselves as simply Local people, or Orthodox, rather than Belarusians (also see: Belarusian minority in Poland).
Pursuant to the Treaty of Riga signed in March 1921 between Poland, Soviet Russia and the Soviet Ukraine (thus ending the Polish-Soviet War), the territories of modern Belarus (part of the Russian Empire) were divided between Poland and the Russian Soviet Federative Socialist Republic. The area that became part of Poland formed the central part of Kresy. In Soviet times, it was called West Belarus as oppose to East Belarus. The new borders established between the two countries remained in force throughout the interwar period, up until the outbreak of World War II. They were later redrawn during the Yalta Conference and Potsdam Conference.
Thousands of Poles settled in the area in the years that followed the Peace of Riga. In the elections of November 1922, a Belarusian party (in the Blok Mniejszości Narodowych coalition) obtained 14 seats in the Polish parliament (11 of them in the lower chamber, Sejm). In the spring of 1923, Polish prime minister Władysław Sikorski ordered a report on the situation of the Belarusian minority in Poland. That summer, a new regulation was passed allowing for the Belarusian language to be used officially both in courts and in schools. Obligatory teaching of the Belarusian language was introduced in all Polish gymnasia in areas inhabited by Belarusians in 1927.
Compared to the (larger) Ukrainian minority living in Poland, Belarusians were much less politically aware and active. The largest Belarusian political organization, the Belarusian Peasants' and Workers' Union, was banned in 1927, and further opposition to the Polish government was met with state-imposed sanctions. In 1935, after the death of Józef Piłsudski, a new wave of repressions was released upon the minorities, with many Orthodox churches and Belarusian schools closed. After the Soviet invasion of Poland in 1939 – portrayed by Soviet propaganda as 'liberation of West Belarus and Ukraine' – some Belarusians welcomed unification with the Belorussian SSR, although attitudes of many changed after experiencing the Soviet terror. From 1939 on, with the exception of a brief period of Nazi occupation, almost all Belarusians previously living in Poland would live in the Belorussian SSR.
After an early period of liberalization, tensions between increasingly nationalistic Polish government and various increasingly separatist ethnic minorities started to grow, and Belarusian minority was no exception. The Soviets were also constantly trying to escalate this conflict, promoting the formally autonomous Soviet-controlled East Belarus to attract sympathies of Belarusians living in Poland. This image was attractive to many West Belarusian national leaders and some of them, like Francišak Alachnovič or Uładzimir Žyłka emigrated from West Belarus to East Belarus, but very soon became victims of Soviet repressions. In 1937–1938 the Soviet NKVD and the Communist Party attempted to eradicate Poles as a minority group[dubious ] in East Belarus during the largest ethnic shooting and deportation action of the Great Terror.
The area of West Belarus was annexed into the Belarusian Soviet Socialist Republic following staged elections soon after the Nazi-Soviet Invasion of Poland in September of 1939. The corresponding terms of the Molotov–Ribbentrop Pact signed earlier in Moscow were broken, when the German army entered the Soviet occupation zone on June 22, 1941. Two years later, at the insistence of Joseph Stalin during the Tehran Conference of 1943, West Belarus was formally ceded by the Allies to the Belorussian SSR following the end of World War II in Europe.
Annexation of West Belarus by the USSR 
Under the terms of the Molotov-Ribbentrop Pact, Poland was divided between the Soviet Union and Germany and was invaded by these countries in September 1939 (see German invasion of Poland and Soviet invasion of Poland).
On October 22, 1939, less than two weeks after the invasion, the Soviet occupational administration organized elections into a National assembly of West Belarus (Belarusian: Народны сход Заходняй Беларусі). The Elections to the People's Assemblies of Western Ukraine and Western Belarus took place under control of NKVD and the Communist Party. On October 30 the National Assembly session held in Belastok passed the decision of West Belarus joining the USSR and its unification with the Belarusian Soviet Socialist Republic. These petitions were officially accepted by the Supreme Soviet of the USSR on November 2 and by the Supreme Soviet of the BSSR on November 12.
West Belarus in its entirety was incorporated into the Byelorussian SSR (BSSR). It was initially planned to move the capital of the Byelorussian SSR to Vilnius. However, the same year Joseph Stalin ordered that the city and surrounding region be transferred to Lithuania, which some months later was annexed by the Soviet Union and became a new Soviet Republic. Minsk therefore was proclaimed the capital of the enlarged BSSR. The borders of the BSSR were again altered somewhat after the war (notably the area around the city of Białystok (Belastok Voblast was returned to Poland) but in general they coincide with the borders of the modern Republic of Belarus.
After entering the Soviet Union, Western Belarusian population, particularly the Poles faced a filtration procedure by the NKVD, which resulted in over 100,000 people deported to eastern parts of the USSR. Presently, Belarus annually celebrates September 17 as a public holiday: reunification of Belarus.
According to the Polish national census of 1921, there were around 1 million Belarusians in the country. There are historians, however, who estimate the number of Belarusians in Poland at that time to be 1.7 million or even up to 2 million. In the 1921-1926 period Poland did not have a consistent policy towards its ethnic minorities. Belarusians in Western Belarus faced Polonization. Belarusian schools, not being subsidised by the Polish government, were facing severe financial problems by 1921.
After the 1930 elections in Poland, Belarusian representation in the Polish parliament was reduced and in the early 1930s the Polish government started to introduce policies intended to Polonize minorities. Use of the Belarusian language was discouraged. Not a single Belarusian school survived by the spring of 1939, and only 44 schools teaching the Belarusian language still existed in Poland at the beginning of World War II.
Refugees from Western Belarus were arrested by Soviet authorities and frequently executed, Kurapaty graves contain many products from Poland - cloths, shoes. The most prominent victim of NKVD was the activist and linguist Branislaw Tarashkyevich. Frantsishak Alyakhnovich was liberated in 1933 after several years in Soviet camps and exchanged toward Tarashkyevich.
See also 
- Republic of Central Lithuania
- Second Polish Republic
- History of Belarus
- Border Defence Corps
- Western Ukraine (disambiguation)
- Alice Teichova, Herbert Matis, Jaroslav Pátek (2000). Economic Change and the National Question in Twentieth-century Europe. Cambridge University Press. ISBN 978-0-521-63037-5.
- Norman Davies, God's Playground (Polish edition), second tome, p.512-513
- (Polish) Stosunki polsko-białoruskie pod okupacją sowiecką (1939-1941)
- Piotr Eberhardt, Jan Owsinski (2003). Ethnic Groups and Population Changes in Twentieth-century Central-Eastern Europe: History, Data, Analysis. M.E. Sharpe. pp. 199–201. ISBN 978-0-7656-0665-5.
- (Belarusian)Уладзімір Снапкоўскі. Беларусь у геапалітыцы і дыпламатыі перыяду Другой Сусветнай вайны
- (Belarusian) Сёньня — дзень ўзьяднаньня Заходняй і Усходняй Беларусі
- (Russian) Congratulation by the President of Belarus on the 70th anniversary of reunification of West Belarus with the BSSR
- Janusz Żarnowski, "Społeczeństwo Drugiej Rzeczypospolitej 1918-1939" (in Polish language), Warszawa 1973
- Eugeniusz Mironowicz, "Białoruś" (in Polish language), Trio, Warszawa, 1999, ISBN 83-85660-82-8
- (Belarusian) Жыцьцё і сьмерць мітаў: Заходняя Беларусь