Well, Prime Directive gives Starfleet a Marine Corps(oh no!, the "M" word!), to act as it's ground forces, and the member planets each have a national guard (army, for clarity) and very small national "training" fleets (system patrol/defence craft) that are under direct control of each planets' government but also act as a reserve for Starfleet. So the national fleets use the same rank structure as Starfleet, and the national guard use the rank structure of the Marines/Ground Forces. The TO&E's are the same also, to make integrating them into Starfleet smoother during a war. ( I know some people out there are a little vehement as to wether or not there are Starfleet Marines, but I always thought it didn't make sense to send SP's to assault a city, it's just not their training, that's what infantry is for. Now I'll duck for cover....)
And we have the Ground Forces as written up in the Dominion War Sourcebook, if you don't have it, download it. It has everything you need to set up Ground Force units, campaigns, whatever... Not to mention a full history of the Dominion War.
So we have Starfleet, who specialize in space combat, and Ground Forces, who specialize in ground warfare... Now I wonder... what kind of skills would a Command Branch guy need to effectively command a real multi-service action? This guy would be at least a Captain/Colonel (have gone to the Starfleet War... I mean Defense College), would make an interesting NPC, just what we need a Star Trek version of Patton, put in cryo chamber labelled "Break in case of War"....
"Retreat?! Hell, we just got here!", annonymous American Marine, WWI
"Gravity is a harsh mistress....", The Tick