Chaffee was commissioned in the cavalry and served with the 15th Cavalry Regiment in Cuba until 1907. Between 1907 and 1911, he was assigned to the Mounted Services School at Fort Riley, Kansas. There, he commanded the mounted detachment supporting students and staff at the Army War College. After attending the French Cavalry School at Saumur (1911–1912), Chaffee returned to teaching at Fort Riley. In 1914 and 1915, he served with the 7th Cavalry Regiment in the Philippines. Chaffee was then assigned to West Point, where he was senior cavalry instructor for the Tactical Department until 1917.
Following the U.S. entry into World War I, Chaffee attended the staff school at Langres, France, and then was an instructor there. Thereafter, he was assigned as a staff officer with the 81st Division and fought with it in the St.-Mihiel and Meuse-Argonne offensives. He ended the war as a temporary colonel.
Following occupation duty in Germany as a staff officer of III Corps, Chaffee reverted to his permanent rank of captain in 1919. He was then an instructor at the Command and General Staff School at Fort Leavenworth, Kansas. He was promoted to major in 1920. After various assignments, he served as the G-3 of the 1st Cavalry Division at Fort Bliss, Texas, between 1921 and 1924. Chaffee graduated from the Army War College in 1925, and from then until 1927, he commanded a squadron of the 3rd Cavalry Regiment. He next served on the War Department General Staff. Promoted to lieutenant colonel in 1929, he was charged with developing mechanized and armored forces for the army. A staunch supporter of mechanized warfare, Chaffee was probably the leading proponent of a separate armored force. In 1931, he joined the new 1st Cavalry Regiment (Mechanized) at Fort Knox, Kentucky, as its executive officer.
Between 1934 and 1938, Chaffee was chief of the Budget and Legislative Planning Branch of the War Department. He then returned to Fort Knox to command the 1st Cavalry Regiment. In November 1938, he was promoted to brigadier general and received command of the 7th Mechanized Brigade, which he led during the maneuvers at Plattsburgh, New York, in 1939 and in Louisiana in 1940, both of which had significant impact on U.S. Army mechanized doctrine.
Chaffee received command of the new Armored Force in June 1940 and thus had charge of the development of the 1st and 2nd Armored Divisions. In October 1940, he took command of the I Armored Corps as a major general, but by that point, he was already ill from cancer. The disease took his life in Boston, Massachusetts, on 22 August 1941. In 1945, the army named its new light tank, the M-24, in his honor.
Mark A. Buhl
Gillie, Mildred H. Forging the Thunderbolt: A History of the Development of the Armored Force. Harrisonburg, PA: Military Service Publishing, 1947.; Johnson, David E. Fast Tanks and Heavy Bombers: Innovations in the U.S. Army, 1917–1945. Ithaca, NY: Cornell University Press, 1998.