In spring 1917, Rau, by this time an elected trade union official in his firm, participated in the attempt to organise a strike against the war. His action led to a reprimand from his employer, and may have hastened his conscription into the army in August 1917. In the army he was trained in the Zuffenhausen-garrisoned Infantry Regiment 126 and deployed to the Western Front as member of a machine gun company. In September 1918 a shell splinter penetrated his lungs. In the following weeks, he was treated in military hospitals in Weimar and in Stuttgart's neighbouring town Ludwigsburg. While in Ludwigsburg, Rau managed to get leave at short notice on 8 November 1918 and joined the in those days developing revolution in Stuttgart.