The holiday celebrates the beginning of the Red Army's liberation campaign in Western Belarus in 1939, which resulted in the reunification of the Belarusian people, divided under the terms of the Treaty of Riga.
The territory of Belarus was divided between the two states as a result of the Polish-Soviet War of 1919-1920. The eastern part of Belarus became the Byelorussian Soviet Socialist Republic, part of the USSR. The western territories of Belarus were annexed to Poland - some of the lands were seized by the Poles during the war, and some of them were agreed under the Treaty of Riga, signed on 18 March 1921.
The Polish state included the territory of more than 112,000 square kilometers with a population of 4.6 million people (according to 1931 data). These lands were given the unofficial name of Western Belarus, which the Polish authorities did not recognize. In official documents, these territories were more often called Kresy Wschodnie.
Western Belarus was a relatively backward agrarian outskirt of Poland. The region was mainly used by the state as a source of raw materials and cheap workforce. The working day in industry lasted for 10-12 hours, salaries were lower than in the other Polish regions. More than 80% of the region's population was engaged in agriculture.
The Polish authorities pursued a policy of polonization and assimilation against Belarusians. They prohibited the use of the Belarusian language in state institutions and banned Belarusian schools. Out of 400 Belarusian schools that operated on the territory of Western Belarus before the Polish occupation, only 16 remained in 1934, and none was left in 1939. The Belarusian press was persecuted. If there were 23 Belarusian newspapers and magazines legally published in 1927, then in 1932 there were eight of them, and only pro-Polish and clerical publications remained until 1937. There were no Belarusian theaters and musical institutions in Western Belarus. The authorities found various reasons to close Belarusian publishing houses, libraries and village reading rooms.
In the late 1930s, the threat of war from Nazi Germany loomed over the entire Europe. The situation required joint actions from the leading countries of Western Europe and the USSR to prevent the war. Realizing this, the Soviet Union made a number of attempts to set up an anti-Hitler alliance with the UK, France and other Western countries in the second half of the 1930s. However, the leading European states refused the offer, secretly seeking to direct Hitler's aggression eastward. The Soviet Union was faced with a choice: either to continue a one-on-one confrontation with Germany's growing military power, or to make attempts to diplomatically ward off the threat. Under such circumstances, the USSR concluded a nonaggression pact with Germany, which was signed in Moscow on 23 August. The document was accompanied by an additional secret protocol on the division of spheres of influence between Germany and the Soviet Union. According to this document, in the event of the liquidation of the Polish state and the division of its territories, the USSR's sphere of influence included the lands of Western Belarus.
In mid-September, German troops approached the territory of Western Belarus and Western Ukraine. The German leadership, referring to the agreement of 23 August, hurried the Soviet Union to act against Poland. Stalin, however, was delaying the intervention. Only when the Polish army was defeated, practically all the original Polish territories were occupied by German troops, and only isolated points of resistance remained in the country, did the Soviet government order the Red Army to cross the border.
