Engagements of the American Expeditionary Forces in France (Gen. John J. Pershing, commanding) in which he participated included St. Mihiel (12-21 September 1918), trench and tank combat against German defensive positions in northeast France, hampered by driving rain, mounting mud, and overstretched supply lines; Meuse-Argonne (26 September 1918 – 11 November 1918) frontal assaults village to village on the Western Front and through the Argonne Forest; and, post-Armistice, the general defense duty recognized as "Defensive Sector." For his service Rennie was awarded the Victory Medal with Service Clasps. Even in active duty in an artillery crew, Rennie was obsessed by the writing bug. His wartime journals contain multiple references to a beginning freelance journalism career, with submissions of poems, short stories, and articles to magazines and newspapers. Rennie was honorably discharged after return to the U.S. in May 1919.