Alas, Star Trek has gone to hell.
Scott Bakula jumped into a Federation starship captain, and had absolutely no idea what to do.
:dubious:
After the end of the second season (TOS), the Romulans supposedly entered into an alliance with the Klingons against the Federation. This meant they were armed with Klingon ships and materiel, which was a vast improvement over their impulse-driven warships (the original birds of prey).
In reality, it probably had more to do with saving money on ship models and opticals. So far as I know, nothing much ever happened with the alliance (at least, outside of the many ST novels, which I’ve made a point of not reading for the last thirty-odd years).
Interestingly, if you watch the remastered edition of “The Enterprise Incident,” you’ll see the Enterprise bracketed by both Klingon D-7s and the old Romulan birds of prey. In the original version, the latter were conspiculously absent. (I guess CGI made them affordable again.)
One thing that has always bothered me (and this is not isolated to Star Trek, or even science fiction) is the use of the word ‘race’ instead of ‘species’. Traditionally, ‘race’ is a semi-arbitrary designation for someone of the same species to designate a difference either in appearance or heritage (a lot of it is arbitrary because genetically, things are a lot more complicated than common usage would lead you to believe, but that’s beyond this discussion). In any case, outside of science fiction and fantasy, we do not even use the term ‘race’ when talking about non-human species (we don’t say that poodles are a different race from Irish setters).
So why is it in fantasy and science fiction that we call unrelated intelligent species as being of a separate ‘race’?
And even more troubling (at least to me), why is it that we accept that humanity is extremely diverse, with many cultures, and yet in so much science fiction and fantasy, these ‘races’ seem to be characterized in such simplistic terms (war-like Klingons, evil Romulans/Orcs, good dwarves/elves, etc.)
I find it to be quite troubling. Either it illustrates a theme, whether intentional or not, of racism or prejudice, or it shows a distinct laziness.
The science-fiction (and RPG) usage makes more sense, actually.
In any case, some Treks do use “species.”
Perhaps it is futuristic PC.
Is not the term “the human race” in common parlance? :dubious: 
Homo sapiens is the species.
You obviously have a lot of free time on your hands… :rolleyes:
Was this ever stated in the show, or just used as a post-hoc explanation?
Yes, saving money, which is why nothing ever happened with it beyond the TV episodes using one ship for both groups.
From a story-telling standpoint, SF is typically using different aliens to examine human issues. As such, simplifying one culture to an entire species is really just filling in for one nation or one culture on Earth. Thus the monocultures of all aliens, versus the diversity of hoo-mons.
There is more diversity in some written SF, where those themes are examined in greater detail.
It was noted specifically in*** The Making of Star Trek***, which predated the start of the third season. It was referenced obliquely in “The Enterprise Incident.”
SPOCK: Intelligence reports Romulans now using Klingon design.
I believe the last time we saw an original Romulan warbird in the non-remastered TOS was in “The Deadly Years.” I haven’t seen the remastered episode, so I don’t know what it contains now.
In The Enterprise Incident, Spock says, “Intelligence reports Romulans now using Klingon design.”
They couldn’t afford more models.
I believe I said that. :dubious:
I was somewhat more specific in answering his question, but I tender my apologies, nonetheless. 
My (very non-canon) take on it was that the scheming Klingons of TOS were soft, civilized city Klingons. I can only presume that they were violently supplanted by a fiercer Klingon race of the steppes or mountains, with manly brow ridges to go with their notions of directness, war, and honor. I imagine that the “civilized” Klingons are kept on a short leash, working hard in the engine rooms, dry-docks, laboratories, and mines. (Yes, I know that this view contradicts rare sightings of Klingon engineers on TNG, who seem to have brow ridges. Maybe he was a “boss” engineer whose main job was to watch the shifty guys doing the real work?)
That still does not explain Kang having a smooth forehead in TOS and ridges in DS9.
Yeah, I know, I saw that comment. Myself, I’m not up on the fine details.
FWIW, any explanation for this mess is going to have holes. I’ll just once again point out that the Klingons show significantly different behaviors going from TOS to later iterations. I suppose that could also be explained by a “retrovirus” (a “retcon virus”, if you will), but frankly, IMHO they seem to have morphed into something very much like a nomadic pre-industrial tribe in command of starships.
I did not like the fan fic episode of Enterprise that wasted time explaining it.
The program was improving and just hit it’s stride when it was canceled from the loser episodes.
The best explanation was simply ignoring it. The real-world reason was “The producers changed their mind about how Klingons look after TOS because they had more money to spend on makeup,” after all. It’s not necessary to give everything an in-story explanation. I’m just glad nobody ever tried to justify TOS’ calling Spock a Vulcanian in the first season but simply a Vulcan later.