Adna Chaffee, Jr. was the son of lieutenant general Adna R. Chaffee, Sr.. Born in Junction City, Kansas on September 23, 1884, he was commissioned a lieutenant of cavalry in 1906 after graduating from the United States Military Academy, and won recognition as the "Army's finest horseman." In World War I he was an infantry major with the IV Corps during the St. Mihiel offensive, and as a colonel, he later served with the III Corps throughout the Meuse-Argonne offensive.
Following the war, he returned to his Regular Army rank of captain of cavalry and became an instructor at the General Staff School and the Army School of the Line at Fort Leavenworth. During the 1920s, he helped develop the armor concepts and doctrine of the future. He predicted in 1927 that mechanized armies would dominate the next war and assisted in the first program for the development of a U.S. Army armored force. Assigned to the 1st Cavalry Division in 1931, he continued to develop and experiment with armored forces, thus becoming the leading American advocate of mechanized warfare.
In 1938 he assumed command of the reorganized 7th Cavalry Regiment, the Army's only armored force. Chaffee battled continuously during the prewar years for suitable equipment and for establishment of tank divisions. With the collapse of the French Army in June 1940, Chaffee's 1927 predictions of the importance of armored forces in modern warfare were confirmed.
Chaffee died of cancer on August 22, 1941 in Boston, Massachusetts.
The M24 Chaffee light tank was later named for him.