Ah, the age old question - are there Starfleet Marines? 
I've been thinking and reading about the subject of ground warfare in Trek (there's a great analysis of it here http://www.st-v-sw.net/STSWground.html). While starships and starship combat are definitely the key to any interstellar conflict and having a starship in orbit gives you a huge advantage over any enemy ground forces, having some ground troops does sometimes seem to be neccessary. Say, you want to take a major sensor or communication post, or rescue prisoners (so you can't simply blow the target from space) and the enemy position is concealed and they are using sensor dampening fields and/or transport scramblers (so you can't just stun them or transport them from orbit). And anyway, we have heard of and even seen UFP ground troops on DS9.
Of course, some of them could have been ship security personel detailed to ground duty. That doesn't really seem enough, though. For small scale missions and raids (especially improvised operations) they could prove enough. But if you're conducting a large scale operation or you have to hold the position longer, firstly, you won't have enough personel, and secondly, you'll be leaving your ships vulnerable in case of enemy attack and boarding. Some independent units are in order, then.
I don't think they're really Marines, though, not in the sense of the modern US Marine Corps ie. a large separate branch with a separate command structure, ranks, uniforms etc. I tend to think their relationship to Starfleet (in terms of size and structure) is more like the one between the Royal Navy and the Royal Marines, even closer. So they are, in essence, separate formations of Starfleet Security personel, with the same ranks etc. as shipboard security, but specifically trained and equiped for ground combat. Maybe there are even dedicated shuttle/runabout units for air/armored support. And Special Ops units.
Actually, while these Starfleet Security Ground Combat Units, or whatever you want to call them, are probably the best and frontline troops, I think most 'grunts' are actually provided by UFP member states' 'National Guards' (which I believe to exist). Fighting in space is pretty much exclusivelly Starfleet's job, but resisting ground invasions and otherwise providing planetside security (and maybe even manning orbital defences) is more of a member state responsibility. So when Starfleet takes a location and needs further troops to hold it and thus relieve it's own units, or even when it needs units specialized for certain conditions (say, Elaysians for low-gravity environments or Andorians for frozen planets etc.) it uses federalized member states' forces.
Comments?
I've been thinking and reading about the subject of ground warfare in Trek (there's a great analysis of it here http://www.st-v-sw.net/STSWground.html). While starships and starship combat are definitely the key to any interstellar conflict and having a starship in orbit gives you a huge advantage over any enemy ground forces, having some ground troops does sometimes seem to be neccessary. Say, you want to take a major sensor or communication post, or rescue prisoners (so you can't simply blow the target from space) and the enemy position is concealed and they are using sensor dampening fields and/or transport scramblers (so you can't just stun them or transport them from orbit). And anyway, we have heard of and even seen UFP ground troops on DS9.
Of course, some of them could have been ship security personel detailed to ground duty. That doesn't really seem enough, though. For small scale missions and raids (especially improvised operations) they could prove enough. But if you're conducting a large scale operation or you have to hold the position longer, firstly, you won't have enough personel, and secondly, you'll be leaving your ships vulnerable in case of enemy attack and boarding. Some independent units are in order, then.
I don't think they're really Marines, though, not in the sense of the modern US Marine Corps ie. a large separate branch with a separate command structure, ranks, uniforms etc. I tend to think their relationship to Starfleet (in terms of size and structure) is more like the one between the Royal Navy and the Royal Marines, even closer. So they are, in essence, separate formations of Starfleet Security personel, with the same ranks etc. as shipboard security, but specifically trained and equiped for ground combat. Maybe there are even dedicated shuttle/runabout units for air/armored support. And Special Ops units.
Actually, while these Starfleet Security Ground Combat Units, or whatever you want to call them, are probably the best and frontline troops, I think most 'grunts' are actually provided by UFP member states' 'National Guards' (which I believe to exist). Fighting in space is pretty much exclusivelly Starfleet's job, but resisting ground invasions and otherwise providing planetside security (and maybe even manning orbital defences) is more of a member state responsibility. So when Starfleet takes a location and needs further troops to hold it and thus relieve it's own units, or even when it needs units specialized for certain conditions (say, Elaysians for low-gravity environments or Andorians for frozen planets etc.) it uses federalized member states' forces.
Comments?
. And I don't think it's even really accurate. Marines are Marines to distinguish them from ordinary land forces, the Army. So, you need an Army first. If you have just one ground force, you won't call them Marines, you'll call them the Army, the Infantry, the Ground Forces or something like that. And we're barely sure Starfleet has even one ground force, let alone a second specialized one (or as Timo pointed, needs it; actually, Timo, I wasn't really discussing Marines as specialized units, I was more using it as a generic term for all Starfleet ground forces, as they ordinarily seem to be used on the net).
) and mobile one.
. Wouldn't need any armor with those...
