Stalin was exiled to Siberia for most of WW1 for his political activities (he was a prominent Bolshevik party member and creator of the daily Pravda newspaper, a key spoke-piece for the Bolsheviks). He was conscripted into service but was found medically unfit for service due to a childhood injury to his arm.

He returned to Petrograd in 1917 and was present for the October Revolution and was a key minister in the Bolshevik party during the Russian Civil War (and a particularly bloodthirsty one). He, along with Lenin and Trotsky, were the primary leaders of the Bolshevik party from when they took power until Lenin's death.

Source: A People's Tragedy: The Russian Revolution: 1891-1924, by Figes.