Semyonov was a private in the 12th Army. following the February Revolution in 1917, he was elected to the army committee by his regiment and joined the Socialist Revolutionary Party. After the Bolshevik seizure of power and resultant Russian Civil War, Semynov was a member of the SR Military Commission. However he was concerned that the leadership was ineffectual and advocated – and organised – a terrorist cell, the Central Battle Unit, to combat the Bolsheviks. He was responsible for the murder of V. Volodarsky on 20 June 1918. However following his arrest by the Cheka in the autumn of 1918, he reversed his political alignment and helped provide information which enabled the subsequent show trial of the SR leadership.