I'm trying to figure out the Star Trek lore (especially around the 2400 time frame) about Starfleet ground forces or marines or anything similar. I keep getting point to a group called Section 31 (I think that's the right number) but they seem a bit too rogue and not really fighters but spies.

Anyone have any idea about Star Trek canon around this topic? Thanks for the help.

MACO

Andor also has marines

Starfleet has marines as well

also Chief Obrien

I understood that the MACOs were only during the Enterprise NX-1 timeframe. Is there any reference to them in STO?

Yes

Can't move for Maco round here

Earth had Military Assault Command Operations (MACO) in the 22nd century, but I don't know if that carried over to the Federation.

The troops on AR-558 seemed to be regular Starfleet officers.

Maco ground equipment and maco space equipment

The ground troops in "Nor the Battle to the Strong" were seen only briefly but had a different uniform, sort of Starfleet-esque but with different shoulder colors. The fanbase, including myself, always calls them marines but they were never referred to on air as such. Certainly could have been in the script or in the script writer's notes or something, not sure where it came from.

I kinda think that normally Starfleet and the Federation would not have a standing army. It would have planetary security forces (some local, some starfleet) plus the security detachments that float around on the starships. However, during a war on the scale of the Dominion War it would kinda have to establish an army. Starships are great, but they can't take and hold ground and in this case you'd need a force capable of taking and holding whole planets, several at a time even. We just never saw them on screen. Couldn't do The Longest Day on DS9's budget, haha.

Whilst I acknowledge the presence of MACOs in ENT, I vehemently dislike the notion of "Starfleet Marines".

My respect to the folks who planted the Stars and Stripes on Iwo Jima, but I feel that any dedicated standing army doesn't sit well with UFP Starfleet's humanitarian/exploratory/peacekeeping force.

Sure, ST Marines are popular in many unlicenced RPGs, but they have never been acknowledged as canon, nor did Roddenberry like the notion of a overtly militaristic Starfleet in the first place (apparently the TOS cast complained that the TWoK "Monster Maroons" were too militaristic).

I'd prefer to think that the Starfleet ground personnel on AR-558 were simply volunteers pulled from regular ship complements.

the MACOs are mentioned in the Needs of the Many book. Some of the other books have suggested that there are Starfleet Ground Forces, as I recall, the people fighting on Ajilon Prime in Nor the Battle to the Strong are referred to as soldiers, suggesting that they are more then just starfleet crewmen, and there is definitely mention of some sort of Special Operations Group. The Uniforms worn by the soldiers on AR-558, and Ajilon prime are referred to as "Surface Operations Blacks" in the books and backstage sources; they fit on over the standard uniform undershirt and the books claim that they provide some additional protection against phaser shots.

This. Roddenberry was adamant that Starfleet was NOT a military organization. Granted, the Dominion War necessitated a "Ground" fleet, but I like to think that was a necessary thing for that time. They created a ground unit, and dissolved it after the war. But I do not believe the Federation has a standing Army/marine/ground force.