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.
In mid July 1918 LSR tried to incite an uprising in areas ceded to Germany by Brest-Litovsk Treaty,[1] so their supporters were involved in the assassination of German ambassador Graf von Mirbach in Moscow, Russian SFSR, on July 6, and Field Marshal von Eichhorn, commander of Army Group Kiev and military governor of Ukraine, on July 30.
Von Mirbach's assassination signaled the beginning of LSR's failed revolt in Moscow in 1918, and was a part of Dual Entente plan to undermine Lenin's power by supporting most powerful opposition after Ataman Kaledin's rebellion in the Don region failed in January 1918. Some LSR became full members of the Communist Party of the Soviet Union.