The Russian Civil War was a multi-party war that occurred within the former Russian Empire after the Russian provisional government collapsed and the Soviets under the domination of the Bolshevik party assumed power, first in Petrograd (St. Petersburg) and then in other places. In the wake of the October Revolution, the old Russian Imperial Army had been demobilized; the volunteer-based Red Guard was the Bolsheviks' main military force, augmented by an armed military component of the Cheka, the Bolshevik state security apparatus. There was an instituted mandatory conscription of the rural peasantry into the Red Army. Opposition of rural Russians to Red Army conscription units was overcome by taking hostages and shooting them when necessary in order to force compliance. Former Tsarist officers were utilized as "military specialists" (voenspetsy), taking their families hostage in order to ensure loyalty. At the start of the war, three-fourths of the Red Army officer corps was composed of former Tsarist officers. By its end, 83% of all Red Army divisional and corps commanders were ex-Tsarist soldiers.
Where did Soviets under the rule of the  Bolshevik first assume power during the Russian Civil War?
Petrograd (St. Petersburg)