Left Socialist-Revolutionaries

:"Not to be confused with Left Socialists (Denmark), Left Socialist Party (Sweden) and Left Socialist Party (Belgium)."In 1917, Russia the Socialist-Revolutionary Party split between those who supported the Provisional Government, established after the February Revolution, and those who supported the Bolsheviks who favoured a communist insurrection.

The majority stayed within the mainstream party but a minority, who supported the Bolshevik path became known as Left Socialist-Revolutionaries. Maria Spiridonova was a prominent leader of this group. They, in effect, split from the main party. The split had not been completed before the Russian Constituent Assembly elections, the first meaningful electoral test between the parties in the peasant soviets a few weeks after the Assembly elections showed the parties had roughly equal support in the peasantry

The Left SR party became the coalition partner of the Bolsheviks in the Soviet Government after the revolution of October 1917. They later resigned their positions in protest at the signing of the Treaty of Brest-Litovsk. The left-SRs were implicated in the assassination of the German ambassadors to the Russian SFSR, Count Wilhelm Mirbach and Ukraine, Hermann von Eichhorn. In 1918 they attempted the failed revolt.

Some left-SRs became full members of the Communist Party of the Soviet Union.