While all these debates over semantics, concepts and continuity in the STAR TREK universe are fascinating to skim/read over, I think we're missing a few valuable points here:
1: The whole basic notion on which STAR TREK is obviously founded is that it is at least 150-400 years into future, so some basic concepts and terminology may have significantly changed. The United Earth Starfleet of ENT and the Federation Starfleet of TOS/TNG/etc. may serve, in some vaguely recognizable fashion as the official "military" of their time. But does that make those organizations militaristic on the 20th or 21st century sense? As with many things in TREK, the answer is left open. The story and the characters are emphasized; the fictional Galaxy they inhabit is not so clear. (Hell, even the story and characters are often vague in many ways.)
2: We never see any organization explicitly spelled out as "marines" or "ground troops" in a purely recognizable "Old Earth" fashion. But we do see uniformed Federation personnel taking orders from Captain Sisko during "The Siege of AR-558
", as if they were functionally serving, in some vague fashion, as "ground troops". Countering this obvious development, though, is the very format of the TREK Universe. It is made clear, time and again, what the size and scope of starships, star fleets, and their hostile encounters would be. "Yesterday's Enterprise" from TNG's Season 3 made it clear that tens of billions had died in the Federation-Klingon conflict. This strongly implies that whole populations had been nuked or otherwise similarly wiped out. While no orbital bombardments were explicitly spelled out (they were in the unrelated TNG 2nd Season outing "Contagion"), it is very strongly implied that combat in that TREK Universe revolves around a battleship-and-bombing model, not one of planetary-scale invasions and occupations. To put it simply, the moving of armies of live troops (or robotic troops) sufficient to invade/occupy a planet would require space vessels of massive bulk. The only species that comes to mind for having ships built for that purpose would be the Borg. (The unseen Jem'Hadar invasion of Betazed during the Dominion War never made sense; even using the largest Dominion battleships would require huge fleets to overwhlem a whole planet.) So the whole starship-capacity-model in the TREK Universe rules out the practical notion of millions of troops invading worlds.
This doesn't rule out a very different model, however...
(3: The whole notion discussed thus far seems to rotate around "marines" and "armies" of combat "soldiers". Perhaps, for sake of argument, we should step back from such a direct, purely up-front militaristic approach.
We know that Federation and Starfleet conduct and structures favor peaceful diplomacy, non-interference, defense-over-offense-where-possible, exploration, science, cooperation, and constructive exchange. While the military aspects of Starfleet are undeniable, it need not be a purely military organization. Perhaps it is best described as a hybrid organization.
Consider that, in both TOS outings "Friday's Child
" and "A Private Little War
", in is made clear that uniformed Federation Starfleet personnel are, from time to time, assigned to planetary exploration. (Lt. James T. Kirk apparently lived with Tyree on Neural; Dr. McCoy also lived among the Ten Tribes on Capella IV. Both of these cases clearly spelled out that Starfleet personnel were assigned
there.) Also, in TOS' "Court Martial
", Jamie Finney urged Capt. Kirk to take a "ground assignment
In "Balance of Terro
r", the Enterprise races to the aide of Earth Outpost 4 along the Romulan Neutral Zone, only to arrive too late. The outpost's sole representative that we see is Commander Hansen, who is apparently a human. What is not clear is what organization Hansen represents. Is he Federation Starfleet? Is Outpost 4 a kind of Federation Starbase? Or is it something else? Is it an Earth installation, leftover from the pre-Federation days of the Earth-Romulan Conflict? We don't know. It's all conspicuously vague, left up to the viewer's imagination.
The same goes for Space Station K-7 in "The Trouble with Tribbles
" and also the Minara Research Station seen in "The Empath
". Are these Starfleet naval facilities, or something else? It's never made clear. While it is apparent there may be civilian presence on any such facilities, it seems possible they could be operated/sponsored by the Federation in general or Starfleet in particular. (After all, the Enterprise responded to check on both Outpost 4 and the research station.)
But if these facilities are Starfleet operations, wouldn't they be naval in their organization? Perhaps not. Starfleet appears to operate space stations, planet-based starbases, and dockyards. Why wouldn't it have a subsidiary devoted to "ground assignments" as well?
Back in 2010, I started a thread about a similar subject "Conjecture: StarFIREs
", to refer to a "ground assignment" subsidiary of Starfleet. Note that Starfleet would be the parent organization. The notion of StarFIREs (or TACTS) would not
be its own organization. Think of this "ground assignment" operation as being part-Navy SeaBees, part-Peace Corps, part-expeditionary force and part-explorers.