In TMP 6, THE UNDISCOVERED COUNTRY, Rene Auberjonois portrayed the character named West, who wore a Starfleet uniform but who held the rank of colonel. In ENT, the introduction of Earth MACOs seemed to echo the notion of some kind of "Marine detachment" aboard a starship. I've occasionally rolled the idea around in my head, trying to think how it would've worked in the post-ENT Federation. Here's the (admittedly vague) conjecture I came up with: StarFIREs - Starfleet Itinerant Regiments and Expeditions. StarFIREs are a subsidiary of the Federation Starfleet. They are, in fact, Starfleet personnel. Starfleet Command's naval structure is the superior organization. StarFIREs exist as either as small units (ship's "Marine detachment") or as mission-specific "ground forces" (like "Sea Bees" or "ground troops", or other expeditionary forces. While is space, StarFIREs can function inside ordinary Starfleet naval protocol, with StarFIRE personnel acting as an on-board-ship/station detachments. Titles and ranks/ratings while in space may tilt toward naval conventions. Once in planetary orbit or upon making planetfall, StarFIRE personnel may switch to a more "army" or "air force" -like conventions. (A petty officer may be referred to as a "platoon sergeant".) Uniforms would be similar to Starfleet naval units, but there would be visible differences to distinguish them. So why haven't we seen more of them? Well, it seems to me that ground operations in most TOS, TNG and VOY eps were about civilian colonists or Starfleet starbases. DS9 did give us a taste of what could have been StarFIREs in "The Seige of AR-558". But it was never made clear what those ground forces were. So maybe the abscence of evidence does not mean the evidence of abscence. More to come.