A bearded Col. Selah H.R. "Tommy" Tompkins at the Juarez Racetrack in 1919, a post Punitive Expedition incident in which the US crossed into Mexico. Known as "Pink Whiskers", the beareded Tompkis was from a distinguished military family. This photo is surprising in that by this time beards were no longer allowed in the U.S. Army.
the Army may allow men to grow beards.
Once very common amongst American soldiers, by 1917 they'd disappeared in junior enlisted men and were uncommon in senior enlisted men, although not wholly unknown. They were still worn by some officers. But they became a casualty of gas during World War One. You can't easily seal a gas mask if you have a beard. So, by wars end, troops were clean shaven as a rule, and in the US case, they certainly were shortly after the US entered the war.
But gas has pretty much disappeared as a weapon of war. And there's an exception, as there should be, for Sikhs in the US military. So, the logic goes, they ought to be allowed for everyone.
The problem remains gas. At least CS gas, which is tear gas. The US has deployed tear gas on the battlefield at least as recently as the Vietnam War. That's not consider a violation of the rules of war, as its not lethal. National Guardsmen certainly risk being exposed to tear gas if they are called out for riot suppression. And poison gas, even if a rarity, still exists as a potential weapon, so that must be taken into consideration, or at least should be.