Size of the U.S. Army, 1914-1920
This includes all military personnel on extended active duty with the Army (Regulars, volunteers, militia, inductees, reserves and National Guardsmen) and U.S. Military Academy cadets. This does not show the actual strength of the American Expeditionary Force (AEF) in France nor does it reflect the total number of men (4,734,991) who served at some time in the military (army, navy, marines) during the war.
The line graph shows the incredibly fast expansion of the army once war was declared and the equally swift demobilization after the armistics of 11 November 1918. Once the U.S. Senate rejected ratification of the Treaty of Versailles on 19 November 1919, there really was no need for an expanded army as the U.S. shrank from playing a more active role in world affairs.
Sources: U.S. Department of Defense, OASD (Comptroller), Directorate for Information Operations and Control, Selected Manpower Statistics, (May 1975), page 23 (https://archive.org/details/DTIC_ADA043053/page/n25)