From San Francisco on memory Alpha, seems the reason was historical, both in-universe...:
Role in diplomacy
The city had a prestigious history as a place of peace. In 1945, the Charter for the United Nations was signed in San Francisco by some fifty of Earth's nation-states.
After World War III, the nations of the world met in San Francisco to ...
In "Where No Man Has Gone Before", the Enterprise picks up a ship recorder from the SS Valiant, a ship that had crossed the edge of galaxy 200 years earlier. The recorder indicated that the Valiant's captain had given an order to destroy the ship -- an order which, judging by the condition of the recorder, must have been carried out. (We don't know whether ...
He is not the first Vulcan in Starfleet.
Star Trek Enterprise by itself puts a nail in the coffin: near the end of the third season, when the ship returns to Earth after having stopped the Xindi attack, T'Pol is formally inducted into Starfleet and given the rank of Commander.
In TOS, there was also a clear indication: in "The Immunity Syndrome", USS ...
TL;DR, Starfleet isn't a military organisation, according to every relevant source both in-universe and out-of-universe.
To start with, I shall quote Captain Picard (from TNG : Peak Performance)
PICARD : Starfleet is not a military organization. Our purpose is exploration.
Lieutenant Scott from Star Trek: Beyond
SCOTT: The Federation. Starfleet. We'...
No, but it's complicated
Please note that there are two types of prefixes described:
The name prefix, followed by ship's name. Indicates the origin/owner of the vessel. Examples:
USS in USS Enterprise
IKS in IKS Rotarran
The registry prefix, followed by ship's registry number. Indicates the type/purpose of the vessel. Examples:
NCC in NCC-1701-D
NAR in ...
In short, the EMH is not "just as competent as a human doctor", at least not until he's had a chance to go beyond his original programming. In the earlier episodes his lack of bedside manner, abrasive attitude and general lack of interpersonal skills are noted by many staff along with his obvious shortcomings as a crewman and inability to enter areas where ...
At first glance this question is easily answered: Gene Roddenberry says it isn't, Picard says it isn't, and so it isn't.
It is not that simple. It is completely dependent on how narrowly you define the word military.
Is Starfleet an armed force? Yes. (any episode where armaments are used)
Is it tasked with protecting the Federation from foreign enemies? ...
Most of us think of Prime Directive as just prohibition of interference with less developed society. But fact is that Prime Directive is a doctrine of total non-interference with the affairs of other species, irrespective of technology level. See Wiki for that.
Also from Wiki
Federation citizens did not need an exception as the Prime Directive
did not ...
In "The Wounded", it wasn't Starfleet who used the prefix code, but the Cardassians after Picard gave it to them when the Enterprise couldn't get to the battle in time. The simplest explanation is that Maxwell was caught by surprise that the Cardassians would have the code in the first place and so was unprepared for the tactic. Note that the trick didn't ...
I hazard that he was not. In the Original Series episode "Immunity Syndrome", we have the USS Intrepid. We are told the Starfleet vessel was crewed exclusively by Vulcans. To have 430-ish Vulcans, from the Captain down to the Assistant Dishwasher, would suggest several of his people were ahead of Spock in processing through the Academy to serve.
From the Memory Alpha article on Starfleet:
The primary purpose of Starfleet from its infancy was to "...seek out
new life and new civilizations," and "...go boldly where no man has
gone before." (This quote is attributed to Zefram Cochrane, the
inventor of warp drive on Earth.) (ENT: "Broken Bow")
Starfleet HQ is not the Pentagon. Starfleet ...
Given that humans are not forced into military service by economic circumanstances like many are now, we have to assume other motivations for why humans would join starfleet. That same motivation would be unfulfilled by replacing human crews with holograms.
That said, we see that the EMH is an early experiment in holographic crew - we see an attempt at a ...
Simply put, money is not the only form of compensation.
We already have a question that covers why people might wish to join Starfleet, so I won't belabor the points of self-actualization and personal satisfaction.
That said, even in a post-scarcity economy, there is a recognizable social strata. People will have their own goals, and will have their own ...
From what we've seen, in universe, Starfleet officers don't change division colors for temporary assignments.
Worf for example:
spent most of his first year on the Enterprise-D as a relief officer for the conn and other bridge stations.
WhenTasha Yar was killed, Worf permanently took over as tactical officer/chief of security and his uniform changed to ...
The problem is, there aren't a lot of non-Starfleet Federation ships to go around, which not only makes it harder to spot the pattern, it makes the producers less likely to have gone to the trouble of coming up with a pattern in the first place.
One unambiguously civilian ship is Kassidy Yates's freighter, the SS Xhosa. However, this ship was not ...
Starfleet tried something like that in Kirk's era (TOS: The Ultimate Computer), and it was a disaster. Large-scale automation may be a lingering taboo, similar to how Earth is unwilling to revisit eugenics even after centuries of scientific and social advancement following the Eugenics Wars.
Some of the societies encountered by Voyager experienced violent ...
In "The Apple" Kirk is clearly making a joke. If Scotty doesn't get the engines going he'll be dead in a few hours. Being fired is the least of his troubles!
As has been pointed out before, although Starfleet isn't a military organisation per se, they do have many of the same trappings including a robust court martial system so his being 'fired' (in the ...
The Federation Starfleet is a military organisation - but that doesn't mean their sole purpose is martial.
Their primary mission is exploration and humanitarian - but they also fight off threats as necessary (the Klingons, the Romulans, the Borg, the Dominion). It is this secondary role as defenders of the Federation that makes them military.
If you're referring to the Earth Starfleet shown in "Enterprise" (e.g. which predates the UFP) then the strongest contender is Doctor Phlox, followed by T'Pol.
Although Phlox is on an "exchange program" he readily identifies himself as the ship's Chief Medical Officer and was in that capacity well before T'Pol agrees to join the Enterprise' crew.
In the TOS-era novel Star Trek Vanguard: Harbinger, captain Hallie Gannon finds herself and her ship the Bombay in a fight with six Tholian battle cruisers. After a short but heavy combat sequence, she rams the ship into one of the attacking cruisers, activating the self-destruct for extra punching power.
I think that would be less under the Prime Directive and more under First Contact procedures.
First Contact procedures have an application, and that's when you're likely to encounter a civilization during exploration. It's not a factor when visiting their homeworld; you know you're a guest there. They're worried about chance encounters in other worlds ...
The Memory Alpha page on Starfleet has a section specifically on its organisation. It describes it as follows:
Direct commanding authority over Starfleet was a power of the
Federation President. (DS9: "Paradise Lost", Star Trek VI: The
Undiscovered Country) In addition, the Federation Council, the
Federation's legislature, often directly influenced ...
The Prime Directive, aka General Order One, states that: "No starship may interfere with the normal development of any alien life or society."
That has often been translated in the facts as forbidding contact with any civilisation unable to reach the stars by themselves, with the possession of working warp technology used as the tipping point, as explained ...
Unknown at this time
I've tried to find a canon / official source of this information on and off since you asked this question. Given that I could find nothing, and given no one else seems to have, I'm ready to declare that we just don't know.
From an out-of-universe vantage point, no storyline has necessitated such an estimate, and so the writers had no ...
Star Fleet makes up a very small portion of the human population. It only attracts the extremely bright, self motivated, high achieving, explorer type individuals.
I suspect a large portion of the human population live more normal lives: stay on earth, raise families, travel to other planet colonies, read books, paint, live in Holodecks, get drunk in bars, ...