Commonwealth, created in 1569. The Russians, led by Ivan III of Moscow, began military conquests in 1486 in an attempt to reunite the lands of Kievan Rus', specifically the territories of Belarus and Ukraine.
The union between Poland and Lithuania ended in 1795 with the partitioning of Poland by Imperial Russia, Prussia, and Austria. During this time the territories of Belarus were acquired by the Russian Empire under the reign of Catherine II and held until their occupation by German Empire during World War I.
Since initial independence
During the negotiations of the Treaty of Brest-Litovsk, Belarus first declared independence under German occupation on 25 March 1918, forming the Belarusian People's Republic. Immediately afterwards, the Polish–Soviet War ignited, and Belarus found itself torn between resurgent Poland and Soviet Russia. A part of Belarus under Russian rule emerged as the Belorussian Soviet Socialist Republic in 1919. Soon thereafter it merged to form the Lithuanian–Belorussian Soviet Socialist Republic. The contested lands were divided between Poland and the Soviet Union after the war ended in 1921, and the Belorussian SSR became a founding member of the Union of Soviet Socialist Republics in 1922. The western part of modern Belarus remained part of Poland.
In the 1920s, agricultural reforms that culminated in the Belarusian phase of Soviet collectivization was set in motion. In the 1930s, the implementation of the Soviet five-year plans for the national economy led to rapid industrialization.
In 1939, Nazi Germany and the Soviet Union invaded and occupied Poland, marking the beginning of World War II. Much of northeastern Poland, which had been part of the country since the Peace of Riga two decades earlier, was annexed to the Belorussian Soviet Socialist Republic, and now constitutes West Belarus. The Soviet-controlled Belarusian People Council officially took control of the territories, whose
