The first unit to receive the M1918 or Rifle, Caliber .30, Automatic, Browning, M1918 was the US Army's 79th Infantry Division. They first employed them in combat on 13 September 1918. It's interesting to note the BAR was used against the Germans by 2nd Lt. Val Allen Browning, the inventor's son. Despite being introduced just before the war ended the BAR made an impact disproportionate to its numbers. It was widely fielded during the Meuse-Argonne Offensive and its capabilities and potential made a significant impression on other Allied troops. It is interesting to note France put in a request for 15,000 BARs to replace some of their Chauchat machine rifles.
Did you know the BAR was originally designed to be carried by infantrymen during an assault while supported by the sling over the shoulder while fired from the hip? This is a French concept called "walking fire". American troops were taught French doctrine during training in Europe during World War I. The BAR replaced the French 8mm Chauchat initially fielded by American troops.
Another interesting bit of trivia is how US Marines briefly took possession of the BAR during World War I. Troops from the First Battalion of the Sixth Marines had sweet talked, begged and stolen BARs from the US Army's 36th Division, offering them Chauchat's in trade. However, complaints from officers of the 36th resulted in the issuance of a command from Marine Lt. Col. Harry Lee on 9 October 1918: All Browning guns and equipment in Marine possession were to be turned in.
In 1938 work was conducted on improving the design. A pistolgrip was added along with a rate reducing mechanism and a bipod, but the Army killed this project and so died the possibility of a truly upgraded model. The M1918A2 was ultimately adopted and served during World War II, Korea and into Vietnam. It had a new rate reducing mechanism, wide footed bipod and the rear sight was recalibrated for the new 150-grain M2 ball load.
The US Army began phasing out the BAR in the late 1950s. It was planned to be replaced by a squad automatic weapon (SAW) variant of the M14. This didn't pan out and the US Army was without a truly portable light machine gun until the introduction of the M60 machine gun in 1957.