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 war, 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.

In 1931, newly promoted Lieutenant Colonel Chaffee was assigned as the executive officer of the embryonic 1st Cavalry Division, where he continued to develop and experiment with armored forces and became the leading American advocate of mechanized warfare.

Chaffee led the unit through the Plattsburg and Louisiana Maneuvers of 1939–1940, where he helped develop Army doctrine for armored and mechanized formations.

[3] In June 1940, Chaffee was appointed the commander of the Armored Force, and given responsibility for integrating all branches of the Army into mechanized warfare.