War was declared in July 1914 and on 5 March 1915, before he had been able to complete his medical studies, Kostja Zetkin was conscripted into the army. He served as a medical officer on the Western Front, participating in the Battle of the Somme, at Verdun and, later, at Rheims. He continued as a medical officer through several promotions, and was awarded the Iron Cross (Class II) on 10 November 1916. After war ended in 1918 he resumed his medical studies, passing his state medical exams in 1923 with a distinction.