First Army is the oldest and longest-established field army of the United States Army. It served as a theater army, having seen service in both World War I and World War II, and supplied the US army with soldiers and equipment during the Korean War and the Vietnam War under some of the most famous and distinguished officers of the U.S. Army. It now serves as a mobilization, readiness and training command.

The American Expeditionary Forces (AEF) was a formation of the United States Armed Forces on the Western Front during World War I, comprised mostly of units from the U.S. Army. The AEF was established on July 5, 1917, in France under the command of then-Major General John J. Pershing. It fought alongside French Army, British Army, Canadian Army, British Indian Army, New Zealand Army and Australian Army units against the Imperial German Army. A small number of AEF troops also fought alongside Italian Army units in 1918 against the Austro-Hungarian Army. The AEF helped the French Army on the Western Front during the Aisne Offensive in the summer of 1918, and fought its major actions in the Battle of Saint-Mihiel and the Meuse-Argonne Offensive in the latter part of 1918.