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?
I think where the disconnect happens is that the overall geopolitical situation in Star Trek is meant to be somewhat analogous to the conditions during the Cold War, but that Romulans and Klingons aren't directly analogous to any specific group or nation. There are comparisons that could be made, but those are left to the audience. They did a similar thing with the Cardassians and Bajorans, where they aren't meant to represent specific groups in a specific conflict, but the audience can clearly draw comparisons to various parts of human history.