American Expeditionary Forces (A.E.F.), the official designation for United States troops serving in Europe in World War I. John J. Pershing, with the temporary rank of general, National Army, commanded the A.E.F. throughout its service. He was ordered to France on May 26, 1917, and was followed shortly by elements that formed the First Division. Eventually 2,000,000 men were sent to Europe. There were 42 divisions, of which 29 were in combat.

The First Army was set up on July 24, 1918, effective August 10, under Pershing's personal command, which he retained until organization of the Second Army on October 15. Lieutenant Generals Hunter Liggett and Robert L. Bullard then took command of the two armies. The Third Army, which was being organized at the time of the Armistice, became the Army of Occupation. The A.E.F. was organized into nine corps, of which seven were in combat, including the Second Corps, serving with British forces. Small contingents of the A.E.F. served in Italy and at Archangel and Murmansk, Russia.

The A.E.F. came to an end in 1919 when United States troops were withdrawn from Europe.