When the United States entered World War I in 1917, Dr. Harry applied for military service but was rejected, since he had had an appendectomy a few months earlier. In 1918, he reapplied to the Army Medical Board, and in July 1918 he was commissioned as captain and assigned to the Neuro-Surgical Institute of New York for training. He resigned his position as director of the Zoo board of trustees and left to join the service. But he was awaiting transportation overseas when the Armistice was signed in November 1918, so he never got to the battle front. He returned to San Diego, where the Zoo board welcomed him back heartily, and he continued his own personal fight on the home front to establish the San Diego Zoo.