Considering the number of witnessed Federation-wide threats, as opposed to the ones merely causing uncertainty about the continuing existence of planet Earth, I'd say the chances of some other defense service existing are a flat zero. Were something else out there, we would have seen it many times over during the Dominion war.
Except the Klingons and Romulans were arguably only an immediate threat to Earth and Earth-local operators in the first place.
The problem with this argument is that ENT Season Four makes it clear that the Romulans were very clearly more of a threat to Vulcan, Andor, and Tellar than they were to Earth. Meanwhile, TOS makes it clear that the Klingons were a long-term rival who threatened Federation border worlds.
Even the Cardassians didn't become a problem until humans started building colonies along the expanding Cardassian border,
Plenty of the Federation colonists seen along the Cardassian border, and plenty of the Maquis, were non-Humans. Colonization along the Cardassian border was a Federation endeavor, not a Human endeavor.
which would seem to suggest that while the Dominion War could be fought along many fronts, the "Earth front" that included Earth, Cardassia and Bajor would have been the primary responsibility of Starfleet
DS9's Dominion War arc, especially "Favor the Bold," make it very clear that Earth is a distance away from the Cardassian border, and that defense of all Federation worlds was the responsibility of the Federation Starfleet, including Vulcan, Andor, Tellar, and Betazed.
if there existed a separate Andorian Blue Fleet or somesuch...
How often was Andoria even MENTIONED during the Dominion War? By TNG's time, it's not even clear whether or not the Andorians are Federation members.
Andor was established to still be a Federation Member in the 24th Century in several DS9 episodes, most explicitly in "In the Cards" -- in which it was established that the Federation Starfleet is responsible for the defense of Andor.
Neither. I was asking about the scope of the fictional universe in which Star Trek takes place,
Let's look at the context of the conversation again:
At the end of the day, it's a question of how big you think the Star Trek universe really is. Is it a place where a handful of small far-flung planets, divided by huge interstellar gulfs, have banded together to find common ground culturally and economically, or is it a place where a handful of solar systems that just happened to be in the same local cluster ceded their autonomy to an Earth-based superstate?
It's a place where over 150 planets, some relatively close and some relatively far-flung, all divided by huge interstellar gulfs, have banded together to find common ground culturally and economically, and thereby ceded their sovereignty, but not their general autonomy, to an interstellar federal superstate whose seat of government is located on Earth, and Earth is one of the planets who have so ceded their sovereignty.
It's very clear that this is a question about the nature of the Federation, not the nature of all of known space.
Which is to say, what "world" are we supposed to be projecting on the Federation when we imagine it? Is it the "world" as defined along anglo-centric terms such as to mean "Our country, and also our allies, and a bunch of people who don't like us" or "the world" in global terms to mean "a bunch of countries that includes us."
Neither. The Federation is something different -- it's more akin to a planet-wide democratic federal state than it is to anything that has ever existed in reality.
This is based on the false premises that accepting a certain amount of centralization in joining a federal system is the same thing as yielding to imperium
It IS the same when it comes to the centralization of one's military, especially if that centralization implies either the disbandment or expatriation of ones' own armed forces.
No, it's not. An empire is a type of polity in which there is a dominant, pre-existing political unit -- the "center" -- which dominates all of the other member polities (the "periphery") for its own benefit. A true federal system is one in which none of the member polities dominate the others, and in which the federal government is one that represents the interests of the entire federal state and balances all of the competing needs of its members.
In particular, with regards to the military -- there is no reason whatsoever to think that Federation Membership involves dissolving local defense forces, or involves compromising the new Member world's defense in the interests of the center.
You become part of a foreign empire the moment you entrust a foreign entity with your national defense,
By that logic, any federal state is actually an empire, and democratic, egalitarian union is impossible.
You don't become part of a foreign empire when you join the Federation. In part, because the Federation ceases to be foreign upon your joining it. The Federation shares the burden of defending your world with you; it supplements rather than undermines your existing planetary defense system.
especially when it is implied that that foreign entity will value its own defense at a higher priority than your own (something Starfleet has been seen to do in nearly every iteration of Trek; they have this concept of "core worlds" which must be held at any cost, the most important of which is always Earth; everyone else is seemingly left to wonder).
When have we ever seen the Federation Starfleet compromise the defense of Member worlds close to the border in favor of worlds closer to the capital?
Hell -- most of the time, it's Earth itself that's facing the threat of destruction, not the rest of the UFP.
The Federation is a democratic union of democratic planetary states, which practices federalism. It leaves a great deal of power in the hands of its Members, but it also has specific sovereign powers itself. And it is not dominated by any one Member -- not even Earth.
Militarily, this appears not to be the case. That's the significant difference here. It could, certainly, be mere coincidence that both Starfleet headquarters, the Academy and all of the major construction and R&D facilities all just happen to be on the same planet as the Federation's headquarters.
All this means is that there's a capital planet where the government and military HQ is housed. That does not mean that Earth dominates the rest of the Federation. Indeed, of the three canonical Federation Presidents, only one of them was even Human (and we don't know if he was from Earth); the others were Grazerite and Efrosian, and no implication was ever made that they were a subordinated caste within the UFP. There's certainly no indication that the Federation Councillor from United Earth in any way dominates the Federation Council, for instance.
Meanwhile, you're overlooking the fact that the canon has established that Starfleet Academy has numerous branch campuses across the UFP, and that Starfleet Command doesn't run every decision through Headquarters -- the Admiralty operates as much out of starbases and HQs away from Earth as it does on Earth. Hell, it was the divisions of Starfleet Command that operated out of Starbase 375 and Starbase Deep Space 9 that controlled most Starfleet operations during the Dominion War. Militarily, Starfleet Headquarters on Earth was almost completely irrelevant.
But it goes beyond coincidence that starbases and ships outside of Sol are overwhelmingly dominated by humans,
Except we don't know this. We have only ever gotten a good look at three 24th Century crews -- 3 out of a Starfleet that DS9 established has dozens of fleets and thousands of ships. This is not enough to have a statistically meaningful sample.
And on top of that, we don't know that all of those Starfleet officers we see running around who look Human actually are Human. Any number of those officers we glimpse but don't get a good look at could be Betazoid, Ardanans, Argelians, Capellans, Elasians, Lumerians, Ullians, Trill, Ramatians, Risians, Deltans, El-Aurians, etc.
And on top of that, even among the Humans, we don't know how many of them actually come from Earth. Lt. Hawk from First Contact, for instance, was established in the TNG novel Section 31: Rogue to be a native of Mars whose family still resented Earth for opposing Martian independence before the Founding of the Federation.
Further, it is very clear that the Federation Starfleet is not the United Earth Starfleet.
That's far from clear at this point.
Yes, it is. The Starfleet of the 24th Century is referred to at numerous points in DS9 as "the Federation Starfleet," not the "United Earth Starfleet;" they talk in DS9 about taking an oath to defend the Federation, not Earth; they always introduce themselves as "Captain X of the Federation Starship Y;" and, most importantly, they refer numerous times throughout TNG, DS9, and VOY to there being six starships Enterprise and never reference the NX-01 -- meaning, they don't count the NX-01, because it was part of the United Earth Starfleet, not the Federation Starfleet.
The canonical evidence does, however, seem to imply that the Federation Starfleet is the UFP's only federal defense organization. It seems that Member worlds maintain their own forces in the same way that U.S. states have their own National Guards and State Defense Forces, but there's no indication that they operate on a Federation-wide scale.
This is problematic insofar as the Federation Starfleet is as highly centralized as it is vis a vis Earth.
You have yet to demonstrate in what manner the Federation Starfleet is highly centralized vis a vis Earth. We've seen numerous Starfleet operations being planned and approved in branches far away from Earth, with no involvement established from HQ whatsoever. Hell, in TOS, we never even saw contemporary Earth, nor was it established that Starfleet Headquarters was on Earth until TMP. (In fact, Franz Joseph's old Star Fleet Technical Manual from the mid-70s depicted Starfleet Headquarters as a space station, not bound to any planet.)
It's becomes more problematic if those local defense forces are depicted as having secondary authority to Starfleet such that the latter can become involved in a situation and tell the locals "We are taking control of this situation." If Starfleet has higher authority than the locals, this again trends towards imperialism.
I'm sure that the Federation Starfleet is a parallel defense agency with no authority over the local agency unless that local agency is called into Federation service by the UFP President -- the same way, for instance, the United States Army has no authority over the Maryland State Guard unless it's called into federal service.
How you handle a foreign threat is a big giveaway for your definition. A federal system prioritizes overall national defense of all its territories; in the event of a foreign threat, it will tend to mass its defenses on the very edge of its territory to prevent the enemy from attacking its outermost/vulnerable areas.
Which is what we saw in the early DS9 Dominion War episodes, before the Dominion was able to break through and enter Federation space. It's also what we saw the Federation Starfleet do in ST09 -- amassing its fleet along the Klingon border, such that the majority of its forces couldn't intervene in time when the Narada was found to have penetrated Federation space and attacked Vulcan and Earth. Then there are the numerous occasions when Earth hasn't even had an appreciable number of starships in orbit or in the Sol system -- V'Ger, for instance. So it's pretty clear that the Federation does exactly that -- try to protect the overall border -- on a standard basis, even to the extent of sometimes leaving its own capital planet relatively undefended.
An empire grows out of a central point, so the response to the foreign threat is to use those outer territories -- and even some of the inner ones -- as buffers to absorb the enemy onslaught or at least keep it at a distance until a compromise can be reached that the central government finds satisfactory (even if the outer territories hate everything about it).
We have never seen this done in Star Trek.
Of course, the Klingons have said EXACTLY this on a number of occasions. "The Federation is nothing more than a 'homo-sapiens only club.' Present company accepted of course." And earlier, "Vulcans are the intellectual puppets of this Federation!" Both the Klingons and the Romulans seem to believe that Earth -- and humans in particular -- are the heart of the Federation, and moreover, they firmly believe that the destruction of Earth would bring the entire Federation crashing down. Maybe they know something we don't?
More likely, they're too hamstrung by their own imperialism to recognize that another powerful state could be federal in nature rather than just a copy of their own system.