Rahn joined the army on 6 January 1915 and transferred to the German Air Force in the spring of 1916. Following flight school at Coslin, he served with FEA 7 for the remainder of the year. Whilst serving with Jasta 19, he broke his nose in a crash on 5 February 1917. During the last two years of the war, he flew with three fighter Staffeln and scored six victories before he was wounded in combat on 17 July 1918. At the time, he was flying a Fokker DR.I, easily identified by its pattern of white diamonds over vertical black and white stripes on the fuselage.