I think what makes this episode so effective is that it doesn't shy away from the grim aspects of war, as one would expect of a mere TV episode from the sixties - especially an episode from a science fiction show. It's all very tense and gripping, like the best war films, such as when Kirk sits down with his key officers for what amounts to a war council. The writers and the actors aren't kidding around here: this is all preparation for a ghastly conflict, potentially the beginning of another years-long battleground. In the final analysis, Kirk's aim is to keep this battleground to just the two ships - but even then it's an endeavor fraught with peril and probable casualties. In fact, I believe this episode holds the record for ship casualties by the end of it. Right at the start of the episode, we see the devastation such battle can produce, in that supposedly well-protected outpost. Then begin the cat-and-mouse war games between the Enterprise and the Romulan ship - it's as exciting as any conflict we've seen on the big screen. Of course, if you're not into war films, you'd have to look for other things to admire in this episode.
What elevates this episode even further is the revelation of just what and who the Romulans are - it's an electric shock of a sort. Now we have even further inter-crew conflict on the bridge of the Enterprise - war does tend to bring out the worst in some people. Due to still nasty attitudes about race in this future, the tension is ratcheted up even further - Kirk has his hands full in this one. I suppose the one weakness in the story is the convenient relenting of the bigotry issue by the conclusion. On the Romulan side, actor Lenard makes his first appearance in the Trek universe as the Romulan commander; he's terrific in the role, the flip side of Capt. Kirk or Capt. Pike, take your pick, done up to resemble Spock more than a little. Surprisingly, his character is not war hungry as we would expect, another eye-opener for this episode. The actor would next return to this universe as Sarek, Spock's father, so he's nothing if not versatile. It's also telling how the first appearance of such characters as the Romulans is usually their best shot, as it is here. They showed up in "The Enterprise Incident" next.