From 1914-1915, Chaffee was posted with the 7th Cavalry in the Philippines, and from 1916-1917 Captain Chaffee was assigned to West Point as the Senior Cavalry Instructor in the Tactical Department . When America entered World War I in April 1917, Chaffee was temporarily promoted to Major and assigned as the adjutant for the 81st Division as it organized at Camp Jackson, South Carolina. During the Major Chaffee served as an Assistant G3 Operations officer in the US IV Corps, and later returned to the 81st Division as the G3 during the St. Mihiel and Meuse-Argonne offensives. Promoted to the temporary rank of Colonel, he became the G3, III Corps at the end of the war, and remained with the corps for occupation duty in 1919.