Belarus said they were free from Germany on 1918. They formed the Belarusian People's Republic. Then the Polish–Soviet War started. A part of Belarus under Russian rule became the Belorussian Soviet Socialist Republic in 1919. Then it added to the Lithuanian–Belorussian Soviet Socialist Republic. Other lands were divided between Poland and the Soviet Union after the war ended in 1921. The Belorussian SSR became a founding member of the Union of Soviet Socialist Republics in 1922. The western part of modern Belarus stayed part of Poland.