The Left SRs rejected the Brest-Litovsk Treaty (later superseded by the Versailles treaty), which imposed onerous terms on Russia in exchange for a separate peace with Russia. The Left SRs exited the Soviet government in protest, and some took up arms against the Bolsheviks. Natanson opposed this course, fearing that a defeat of the Soviet government would spell the end of the revolution and usher in a counter-revolution. He founded the Party of Revolutionary Communism, which supported the Bolsheviks and eventually merged with the Communist Party of the USSR. Natanson was a member of the Presidium of the All-Russian Central Executive Committee. However, Natanson's opposition to the anti-Bolshevik uprising of the Left SRs did not imply whole-hearted acceptance of the Bolsheviks' policies. In particular, he objected to Lenin's decision to ban all other political parties in the Soviet Union. In 1919, due to fear of arrest, he once again went into exile. “I have complete discord with Lenin ... I no longer believe in Lenin,” he told his relatives.