He later enlisted in the US Army and as a captain in the Intelligence Division, General Staff, of the American Expeditionary Force, Siberia. He was stationed in Chita and had numerous encounters with Grigory Mikhaylovich Semyonov, the "ataman" of Baikal Cossacks. Moore was honorably discharged on 17 March 1919. The Emperor of Japan awarded him the Order of the Rising Sun 5th Class for his service as part of the Allied forces in Russia. Moore documented his observations of Allied intervention in the Russian Civil War in his book "Siberia To-day" and became a widely cited authority on Japanese and Russian relations for many decades to follow.