 Franck enlisted in the German Army soon after the outbreak of the First World War in August 1914. In December he was sent to the Picardy sector of the Western Front. He became a deputy officer (offizierstellvertreter), and then a lieutenant (leutnant) in 1915. He was awarded the Iron Cross, Second Class, on 30 March 1915, and the city of Hamburg awarded him the Hanseatic Cross on 11 January 1916. While in hospital with pleurisy, he co-wrote yet another scientific paper with Hertz, and he was appointed an assistant professor in his absence by Frederick William University on 19 September 1916. Sent to the Russian front, he came down with dysentery. He returned to Berlin, where he joined Hertz, Westphal, Hans Geiger, Otto Hahn and others at Fritz Haber's Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry, working on the development of gas masks. He was awarded the Iron Cross, First Class, on 23 February 1918. He was discharged from the Army on 25 November 1918, soon after the war ended.
