Is this my nerdiest post? Yes. Anyway,
The Star Trek wiki had something interesting...
Paul Schneider modeled the Romulans on the ancient Romans, naming the species' homeworlds after the mythical founders of Rome, Romulus and Remus. "It was a matter of developing a good Romanesque set of admirable antagonists that were worthy of Kirk," Schneider related. "I came up with the concept of the Romulans which was an extension of the Roman civilization to the point of space travel, and it turned out quite well." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 34) D.C. Fontana reckoned that Schneider basing the aliens on the pre-existing Roman civilization was the cause for the writer receiving insufficient credit for creating the Romulans. ("Balance of Terror" Starfleet Access, TOS Season 1 Blu-ray) Gene Roddenberry, interested in ancient Rome himself, approved of the initial depiction of the Romulan species. "He loved Paul's having endowed the enemy-Romulans with the militaristic character of the ancient Romans," wrote John D.F. Black and Mary Black. (Star Trek: The Magazine Volume 2, Issue 11, p. 19) Roddenberry's original concept of the Romulans, however, was that they represented 1960s' Chinese Communists.
Yes, Romulans are somewhat based on the Roman Empire and are xenophobic conquerers, but Westerners often ignorantly attribute these traits to communist countries anyway.
Then there is the Klingons, again, the Klingons are nothing like communists, (they're a patriarchal empire) and yet I've seen people say they were based on the Soviets.
So what do you think?
Yeah I was going to say, the parallels of the enemy empires in TOS Star Trek are just there for plot reasons. Klingons were just brown foreign people who might as well represent Japan in WW2 or whatever orientalist stereotypes the writers could think up. They're just these foreign brown people who have a culture around vague concepts of honor and duty as a contrast to the Enterprise who are more fueled by curiosity and helping or whatever. The Romulans in their appearances in TOS were almost the same way. Most of their plot elements were copy and pasted from things like WW2 submarine movies. If any of them were supposed to strictly represent communism, it didn't work out that way, because TOS is very adamant that all of humanity is working together in spite of political or national differences (unless you're a woman, then you can still be on the receiving end of bigotry sometimes).
It can't be understated how powerful Star Trek was by showing an Asian man or a black woman being a respected officer on a spaceship, in full color in 1966