In the years leading up to World War I, Reilly served in Asia and Europe, and he also started writing a weekly military column for the Chicago Tribune. Reilly resigned his commission on January 8, 1914. He then served in British and French ambulance units. When America entered the war in 1917, Reilly, by then a colonel, had assumed command of the 149th Field Artillery Regiment. 42nd Division. His regiment saw combat in France, and soon became known as "Reilly's Bucks." For his actions, he was awarded the Army Distinguished Service Medal in 1919. His citation reads, "Through his tireless energy and technical skill as an artillerist, his regiment gave most effective assistance to the Infantry which it supported."
Post-war life
After the war, Reilly became a brigadier general in the Officer Reserve Corps, as well as a well-known speaker, writer, journalist, and editor on military affairs. He served as a war correspondent, covering conflicts in Poland, Spain, Albania, and France. He edited the Army and Navy Journal from 1921 to 1925. He wrote several books, including Why Preparedness?, based on what he had seen on Europe's eastern and western fronts in 1914 and 1915; America's Part ; and Americans All: History of the Rainbow Division, which described the division's military actions, including stories about soldiers and officers from private to general. In 1922, he helped found the Reserve Officers Association and served as its first president. Today, the association has a scholarship named after him. The scholarship was temporarily suspended in April 2009 but has since returned to active use. In 1938 Reilley was living near Paris and visited Spain as an observer during the onging Civil War.
Are Our Young Men to Have a Chance?: Blitzkrieg, Its Political and Economic Challenge. Civilian Military Education Fund, 1940.
The World War at a Glance: Essential Facts Concerning the Great Conflict between Democracy and Autocracy. Chicago: Laird & Lee, 1918.
Why Preparedness; The Observations of an American Army Officer in Europe, 1914-1915. Chicago: Daughaday and Company, 1916.
Legacy
Reilly amassed a large personal library; several hundred volumes and documents were stored at ROA headquarters in Washington, D.C. ROA, subsequently, gave most of the collection to the Pritzker Military Museum & Library in Chicago, Illinois, where it is now a non-circulating named collection, Henry J. Reilly Memorial Library.
