You know, the same people that were deployed in large numbers on Earth when Starfleet was preparing for a possible Dominion invasion.
Except that in reality those troops were there only to prepare for an uprising of civilians when Leyton grabbed power.
Did the troops themselves know this? Did they wonder when they were deployed in riot gear only?
In different circumstances, would they have fought the Jem'Hadar in the battlefield, donning phaserproof coveralls? Or would somebody else have done it for them?
The existence of a special branch for ground combat is speculation only. But in the TNG context, it's also very weak speculation. Starfleet branches just plain aren't that "special": the very same guys who pilot giant starships may fly small craft like aces the next day, or plot ground maneuvers, or fix warp cores, or decipher a scientific enigma and break an alien lock.
Case very much in point, Miles O'Brien. A random guy from a random starship pressed to landing party duty (Setlik III backstory from "Paradise"), learns to fix transporters, next acts as Tactical Officer for the ship, then fights as a hardened infantryman in massive ground action (Setlik III backstory from "Empok Nor"), next seen at Engineering of another ship ("All Good Things..."), then pilots the battle section of the ship ("Encounter at Farpoint"), then does internal security ("Lonely Among Us") and transporter ops (rest of TNG), then manages an entire space station and flies runabouts in combat.
In this environment, it would be very difficult to argue with a straight face that the guys doing MP duties are
not qualified to take the point in an infantry assault or a special ops raid, fly a hopper into battle, direct an artillery strike, or gut a Jem'Hadar with a pen knife ("What is a pen?" "Never mind, what is a knife?").
Sure, some will be better at it than others. Some will specialize in a thing or three. But an entire separate organization with separate ranks and leaders? Busted. Our only hope for a "dedicated" ground fighter unit, the AR-558 posse, was led by naval-ranked officers.
Shipboard infantry is a go. But in the TNG environment, it's indeed shipboard infantry, in the sense of the
old naval infantry that consisted of ships' crews issued a sword or a rifle...
Timo Saloniemi