Upon receipt of his doctorate, Herzfeld volunteered for service in the Austro-Hungarian Army. World War I broke out shortly thereafter and he served until 1918, rising to the rank of first lieutenant. Herzfeld's thesis advisor Hasenöhrl was called to serve during World War I and was killed at the front.Mehra, 2001, Volume 5, Part 1, p. 226. During his tenure in the military, Herzfeld published six papers on statistical mechanics applied to problems in physics and chemistry, especially to the structure of matter – gases, liquids, and solids.
