Balance of Terror is like a Chipotle burrito: Sinfully yummy...but so stuffed with ingredients that it's close to bursting open like Kane after his run-in with the facehugger. Not only is Balance of Terror a thinly disguised WWIIU-boat versus destroyer yarn, it's also a cautionary tale about racism. If that weren't enough, there's also a message thrown in about the futility of war: War is bad, mkay?
Romulan Bird-of-Prey, the 23rd century's answer to the U-boat
As in many of the early Star Trek episodes, we're treated to the spectacle of Spock using logic to justify kicking some ass; in this case, he points out that showing weakness in the face of Romulan aggression will only result in interstellar war. Thus, to avoid war, the Enterprise crew must pursue and destroy the Romulan Bird of Prey that has encroached upon Federation space.
Logic dictates, captain, that we open up a can of whoop-ass on the Romulans
After his turn as Mr. Sensitivity in The Conscience of the King, Spock reverts to type as the coldly logical Vulcan, cutting the bigoted Stiles short during his clumsy (and roundabout) attempt at an apology for his earlier racism by pointing out that Spock saving his life was dictated entirely by Stiles's value to the Enterprise's crew as a highly trained navigator. Or maybe Spock being short with Stiles had nothing to do with logic and everything to do with him being peeved by the latter's bigotry.