According to the official biography, when the Germans occupied the town in the spring of 1918, Ostrovsky ran errands for the local Bolshevik underground. In July 1918 he joined the Komsomol and the Red Army in August. He served in the Kotovsky cavalry brigade. In 1920 he was reportedly wounded near Lviv and contracted typhus. He returned to the army only to be wounded again and was demobilized on medical grounds.