No, it was the direct quotes by Scott, Picard, Jonathan Archer, Admiral Gardner and Erica Hernandez, as well as the stated intention by Gene Rodenberry, that lead to this belief being popularized. But even without those very CLEAR lines of dialog, there's inescapable fact that Starfleet doesn't ACT like any recognizable military organization. They don't salute their superiors, they don't rigidly enforce the chain of command, they are notoriously lax in both discipline and battlefield readiness, and they eschew any of the traditional martial philosophy one expects from people who are -- and know themselves to be -- professional soldiers.
Star Trek doesn't exist in a vacuum. Many of its viewers are veterans themselves, and even the ones that aren't are bombarded by images of ACTUAL military organizations throughout the genre of science fiction. Starfleet isn't even TRYING to present itself as a military organization, which was actually one of the interesting things about the introduction of MACO during Enterprise: those guys WERE the military, and they knew it, and more importantly, Starfleet knew it.
Just point out to me any modern naval force that would allow any of the following actions:
1) A senior officer reports for duty every day for four and a half years without every putting on a uniform
2) Officers in the same chain of command openly fraternizing and/or marrying each other
3) Resign from the service with only a verbal notification, and be reinstated just the same, no questions asked
4) Steal a shuttlecraft -- or even an entire starship -- with the intention of going AWOL without being executed for it
5) Shoot your superior officer just because he's annoying (also without being executed for it)
6) Refer to a superior officer -- particularly your commanding officer -- by first name while on duty in full view of junior officers and/or cadets
7) Get publicly shitfaced while on active duty
8) Leave your post in the middle of a crisis to rescue your son/daughter/girlfriend/mother from some random problem
9) Leave your post without authorization to murder a politician from an allied country
10) Threaten to kill your superior officer just because he calls you a coward (and then have him come and apologize for it later!)
There's "military discipline." There's "military discipline is definitely lax in this unit." And then there's fucking Starfleet.
Enforced sporadically at best... apparently you only have to stand at attention when your superior is pissed.
Seriously? They almost NEVER do. In fact, that's one of the things that's always kind of bugged me about their lack of combat readiness: it takes them ENTIRELY too long to dial up anything like an adequate response and half the time it's something like "Target their weapons array only" or "Fire a warning shot, just enough to get their attention." (Seriously, Janeway? These guys are ALREADY SHOOTING AT YOU, how much more attention do you need?)
That wasn't the case in the 1960s. Interestingly, it (literally) isn't the case for the Colonial Fleet in BSG either, since Bill Adama has the authority to unilaterally authorize the release of nuclear weapons.
It IS still the case for the Russian Navy, though. (incase you were wondering).
Actually that question is kind of backwards. Why is it so important to see Starfleet as a military organization and why is it so hard to accept that Picard, Archer, Forest, Hernandez and Kelvin Scotty said it isn't? Starfleet NOT being a military doesn't contradict any of its known properties.
Can, and HAVE. Most such shows usually get canceled (or at least viciously mocked) because nobody believes a military organization would let that kind of shit slide.
Troi
FIFY
It is not, however, VERBAL. You can't just take off your badge and tell your CO "I quit!" and walk off the ship an hour later (or in Scotty's case, half an hour later). You SURE AS HELL can't verbally tell your C.O. "I unquit!" and have him give you your old job back just for the hell of it.
I mean, obviously you can in Starfleet. Try that bullshit in the U.S. Navy and you'll be swabbing the decks with your eyelashes for a year.
James T. Kirk stole a fucking STARSHIP, violated a direct order from a flag officer, and sabotaged the engines of ANOTHER starship in the process. He's lucky they didn't prosecute him for PIRACY! And yet with all that he did in that entire incident, he comes back to Earth and is facing judgement from a CIVILIAN official for, "Nine violations of Starfleet regulations." And they DISMISS the charges, just because Kirk's so awesome. Noticeably, the exact same thing happens in TUC, when Starfleet decides to let the Enterprise crew completely off the hook for violating their orders, violating Klingon space, violating the neutral zone, showing up at Khitomer even after being specifically ordered not to, AND for an armed incursion at a diplomatic function in order to arrest several superior officers, all of that on the testimony of a single witness.
There's a REASON military organizations do not take that kind of nonsense lightly. They let Kirk off the hook, then the entire concept of "military discipline" becomes more of a "suggestion" than an expectation.
Which kind of explains this conversation a hundred years later:
REMMICK: Very original, Captain. But how did that child acquire access to a shuttlecraft?
RIKER: Kurland is a highly qualified Enterprise Academy candidate, fully trained in many areas including shuttles.
REMMICK: And did this full training include discipline?
PICARD: Mister Remmick, young men sometimes make rash choices. Which is why Mister Kurland will receive a strong refresher specifically in discipline
Really??? A "strong refresher in discipline?"
You have any idea what would happen if an ROTC candidate stole a helicopter with the intention of running away to a foreign country? I'll give you a hint: it probably wouldn't be a "strong refresher in discipline."
It happened to Worf.
And there wasn't.
Between officers of the same or similar rank. I challenge you to show me an example of an O3 or an O4 who could get away with referring to an Admiral by first name on the bridge of his own flagship.
... where it is considered to be a major problem.
Not so much in Starfleet, where it's just really really funny.
... usually followed by a long speech by the C.O. about needing to stay focussed on the mission, ones responsibility to your fellow soldiers, and the need not to put your personal matters before duty. OR, if it's a really well-written show, with the C.O. actually helping with the rescue and then reprimanding his subordinate after the fact for letting the situation get that far out of control.
Worf AND Riker both leaving the bridge to save Alexander from a burning aquarium? My suspension of disbelief may have military discipline, but that storyline does not.
Duras.
Jesus Christ, he wasn't DEMOTED for that shit. Picard basically told him "The Klingons aren't pressing charges, so you're off the hook... but I really really mad at you." WHAT?!?!
I can, in fact, imagine ANY warrior-race guy saying this to Picard and getting away with it. i can imagine Shran saying it. I can imagine Tomalok or Admiral Jarok saying it. I can even imagine Urdnox Wrex sying it... but then, Urdnot Wrex aint military, and there's a 33% chance of Commander Shepard actually blowing his head off in response.
But that's kin of besides the point: can you imagine Klaus Wennemann saying that to Jürgen Prochnow? (I mean, WITHOUT getting immediately shot in the face?) Can you imagine Denzel Washington saying that to Gene Hackman on the Alabama?
Star Trek just isn't that kind of drama. That is to say, it is not a MILITARY drama, not even stereotypically so.
They can.
DUKAT: Are you telling me that one of the most heavily armed warships in this quadrant is now in the hands of Maquis terrorists? Do you have any idea what kind of response this will provoke from the Central Command?
SISKO: They'll probably want to send ships into the Demilitarised zone to conduct a search. We understand that. But any entry into the zone must be a joint operation between Cardassia and the Federation.
DUKAT: The Central Command will not be interested in any joint operation, Commander. They won't believe your story about transporter duplicates and security failures. They will assume that Starfleet wanted the Maquis to have the Defiant, and they will respond accordingly.
ODO: And what does that mean, exactly?
DUKAT: It means they will seize this opportunity to finally eliminate the Maquis. They'll send a fleet into that zone.
SISKO: Starfleet will cooperate in a search for the Defiant, but they will not allow the Central Command to use this as an excuse for a full-scale invasion.
DUKAT: Of course not. So you will send your own fleet into the zone to protect Federation colonies. But at some point the two fleets will meet, tensions will rise, nerves will fray, and someone will make the tiniest mistake.
SISKO: And we'll have a war on our hands.
So "What's up with that" is that the DMZ is a zone the Cardassians have promised not to place any of their forces in or near, in exchange for Starfleet not doing the same. Which is the WHOLE REASON the Maquis were pissed off about this treaty:
HUDSON: There is no risk, Ben. None. I have been dealing with the Cardassians most of my professional life, and believe me, they are very pleased with what they got from the treaty. They are not about to risk all they've gained by sending forces into the Zone. They're much too slick for that. If I were you, I'd be very concerned with security along the Bajoran border.Basically: Starfleet told them "keep your military out of the DMZ and we'll let you keep those colonies" and the Cardassians responded with "Yes, of course! We'll gladly keep our military out of those colonies! Can't say we can keep our weapons out of there, though... second amendment and all that."
It's actually kind of interesting: from what we've seen, the Federation Government empowers Starfleet to operate as its legal representative in both a diplomatic AND criminal justice capacity. This is why Kirk, of all people, is empowered to pass judgement on Khan and deliver a legally binding sentence, and can open and close proceedings on the prosecution of Harry Mudd. Jean Luc Picard actually has the power to negotiate treaties with people -- as we see him doing in "Encounter at Farpoint" and can represent the Federation in diplomatic first contact situations to the point that his recommendations carry a huge amount of weight.
And this circles back to Kirk's trial in TVH: why is the PRESIDENT OF THE FEDERATION reading Kirk's charges before the entire council? Primarily it's because of the seriousness of the charges, but mostly it's because Starfleet REPRESENTS the Council in a very real sense, and therefore answers directly to it.
So in answer to your question:
By being the legal representative of the legislature that controls it and equivalent, in a way, to an entire branch of the Federation government. Or at least, empowered to act like one.
And it yet those engagements DON'T have serious consequences in Star Trek...
You never wondered why that was?
All these don't prove that Starfleet isn't a military service. It proves that Starfleet is a military service lacking in discipline.