Here's more than anyone needs to know...
In general - Cavalry favored moustaches - big, drooping ones like you see on the 'Old Bill' statue at Frot Riley and on the cover of 'Armor' .
The style harkens back to European horse soldiers - notably English and French Cavalry and Dragoons.
Early ones were generally described as 'Dragoon' moustaches and were pretty impressive - reaching serious length and waxed.
Smaller ones - the big non-drooping-type - were known as 'Guards' moustaches.
Infantry - when not clean-shaven - favored beards.
Facial hair went with the times, for the most part - with the military's (and society's)general 'acceptance' dying out as better razors were developed.
Eventually - the Gillette folks came up with their replaceable-bladed 'Safety Razor' - making it far easier for a man to shave himself - as opposed to having a barber do it.
'Way back when' - barbers were sometimes the only one who 'could' shave a beard - many men not having a 'facility' with the straight-edge razor, so shaving was a sometime thing and difficult unless one lived in town.
By the time of the Mexican Border Expedition and WWI - the far smaller moustache was seen on older soldiers - but the 'look' of the military was clean-shaven.
The Marines - like the other Services - had their own idiosyncracies - but in general - if you see a post-WWI Marine with facial hair beyond a moustache - he's overseas.
The Navy often would permit sailors to grow out their beards while on a Cruise - most notable with submariners - but I'm not certain that that's still a policy.
AR 670-1's old policy stated that the moustache 'could not extend beyond the fullest portion of the upper lip'.
That meant a 'trooper could grow a pretty good-sized 'stache, and the best examples of that are photos of chopper jockeys, doorgunners and crewchiefs during Viet-Nam.
And 'all' were 'legal'...
Imagine that...
Of course - this was also back when Guard/Reserve soldiers were authorized wigs...
This was back in the days when the AR was printed on a slick-paper 'magazine' format.
When they printed it later on - on the 'newsprint' paper - thanks to the many changes happening in the latter '70's - the moustache became smaller - 'not to exend beyond the corners of the mouth' and the information was difficult to locate - being hidden deeply inside the changes.
Still - certain 'styles' remain.
In SOCOM, you'll see more moustaches - thanks to their missions that value assimilation into a population where facial hair is common to the males.
It's called 'relaxed grooming standards' and is a privilege that's frequently abused.
Aviators will still sport one, from time to time - as will some Infantry.
Alas - thanks to the ever-present threat of NBC warfare - beards are no longer seen, since a good seal to the mask can't be achieved.
Vaya,
Scouts Out!