Yeah, I'm sorry, but the concepts of "pure sci-fi" and "Star Trek" stopped sharing the same neighborhood after the first season of TOS. Therefore I offer:
The Corbomite Maneuver
What Are Little s Made Of?
Balance Of Terror
Ever see any WWII set submarine films?
As a matter of fact,
The Enemy Below is one of my favorite WWII movies.
I picked BoT because, unlike most of you here, my defintion of pure sci-fi doesn't include 20th century space probes evolving into superbeings (Changeling and ST:TMP), super ESPERs (Gary Mitchell and the Talosians) or anything else I would consider epic sci-fi. To me, pure sci-fi is about the characters using technology derived from current day scientific knowledge (spacecraft, robotics, directed energy weapons, etc...)
But you don't like my choice. Fine. I'll replace it with:
A Taste Of Armageddon
Happy?