 During the First World War in 1918, Müller was called up to serve in the 18th Foot Artillery Regiment Kirchenhain on the Western Front in France.Marked by his experiences during this time, he became a pacifist and remained so for the rest of his life.Showing something of a Schwejkian streak, Müller often took to drawing when on duty, and he was soon given the task of looking after the regimental horses.