13 Times Star Trek Broke Its Own Prime Directive
9. A Private Little War - Original Star Trek
This will be the last entry here that's in any way genuinely defensible, since the entire episode is both an allegory for the Vietnam war as well as a metaphor for how doing the right thing and protecting the most people more than likely involves doing some things you know are wrong.
When the Enterprise lands on the planet Neural, Kirk discovers that several tribes are being wiped out by a neighboring tribe that's using more advanced weaponry being provided to them by the Klingons.
Not seeing another way out of the situation, Kirk reluctantly teaches the peaceful defending tribes the concepts of war and battle strategy, and provides them with weapons equal to their enemies to defend themselves. They manage to defeat the enemy tribe, but Kirk takes no solace in this victory. And why should he? He's just demonstrated WHY the prime directive is in place. And he's taught a peaceful people ow to make and use advanced killing machines. None of this is going to end in good in the long run.
There wasn't much he could do, but that doesn't make what he ended up doing any more right.