When the German–Soviet War broke out in June 1941, he volunteered for the Red Army and used his knowledge of German to serve as a propaganda officer and an interpreter. He was tasked with subverting and indoctrination Germans, and on one occasion persuaded the German garrison of Graudenz (Grudziądz) to mutiny. When he entered East Prussia with the Red Army throughout the East Prussian Offensive, he sharply criticized the atrocities against the German civilian population and was arrested in 1945 and sentenced to a ten-year term in the Gulag for fostering "bourgeois humanism" and for "compassion towards the enemy". In the sharashka Marfino he met Aleksandr Solzhenitsyn. Kopelev became a prototype for Rubin from "The First Circle".