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World War II

In September 1939, the Soviet Union invaded and occupied eastern Poland, following the German invasion of Poland two weeks earlier which marked the beginning of World War II. The territories of Western Belorussia were annexed and incorporated into the Byelorussian SSR. The Soviet-controlled Byelorussian People's Council officially took control of the territories, whose populations consisted of a mixture of Poles, Ukrainians, Belarusians and Jews, on 28 October 1939 in Białystok. Nazi Germany invaded the Soviet Union in 1941. The defense of Brest Fortress was the first major battle of Operation Barbarossa.

The Byelorussian SSR was the hardest-hit Soviet republic in World War II; it remained under German occupation until 1944. The German Generalplan Ost called for the extermination, expulsion, or enslavement of most or all Belarusians for the purpose of providing more living space in the East for Germans. Most of Western Belarus became part of the Reichskommissariat Ostland in 1941, but in 1943 the German authorities allowed local collaborators to set up a client state, the Belarusian Central Council.

During World War II, Belarus was home to a variety of guerrilla movements, including Jewish, Polish, and Soviet partisans. Belarusian partisan formations formed a large part of the Soviet partisans, and in the modern day these partisans have formed a core part of the Belarusian national identity, with Belarus continuing to refer to itself as the "partisan republic" since the 1970s. Following the war, many former Soviet partisans entered positions of government, among them Pyotr Masherov and Kirill Mazurov, both of whom were First Secretary of the Communist Party of Byelorussia. Until the late 1970s, the Belarusian government was almost entirely composed of former partisans. Numerous pieces of media have been made about the Belarusian partisans, including the 1985 film Come and See and the works of authors Ales Adamovich and Vasil Bykaŭ.

The German occupation in 1941–1944 and war on the Eastern Front devastated Belarus. During that time, 209 out of 290 towns and cities were destroyed, 85% of the republic's industry, and more than one million buildings. After the war, it was estimated that 2.2 million local inhabitants had died and of those some 810,000 were combatants—some foreign. This figure represented a staggering quarter of the prewar population. In the 1990s some raised the estimate even higher, to 2.7 million. The Jewish population of Belarus was devastated during the Holocaust and never recovered. The population of Belarus did not regain its pre-war level until 1971. Belarus was also hit hard economically, losing around half of its economic resources.