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 Soviet rule became the Belorussian Soviet Socialist Republic in 1919. Then it added to the Lithuanian–Belorussian Soviet Socialist Republic. 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 as a result of the Treaty of Riga.