At the outbreak of the First World War in the summer of 1914 Ernst Reuter was at first active in the pacifist "New Fatherland League" which he co-founded and whose aim it was to lead to a rapprochement between the enemies. In 1916 Ernst Reuter was conscripted into military service. In August of the same year he was severely wounded and became a prisoner of war in Russia.
In Russia Ernst Reuter witnessed both revolutions of 1917. The overthrow of the Tsar and the dismantling of the old system encouraged Reuter's political radicalization and led to him joining the Bolsheviks. During his captivity he acquired organizational skills and knowledge of the Russian language, which made him interesting for the Communist Party.
Thus, in April 1918 he was appointed people's commissar of the Volga German Republic. His work at the Volga aimed at maintaining public order among the Volga Germans according to communist ideas and at supporting the food supply for the capital. However, his work lasted only a few months. Already at the end of 1918 he returned to Germany. He became a member of the newly-founded Communist Party (KPD) and was elected its Secretary General in 1921.