What episode features the pacifist Organians stopping a Federation-Klingon war?

The episode featuring the pacifist Organians stopping a Federation-Klingon war is “Errand of Mercy.”

Captain Kirk and Spock travel to Organia after the Federation and Klingon Empire enter open conflict. The Organians appear to be a peaceful, technologically primitive population unwilling to resist Klingon occupation or support the Federation’s use of force.

Klingon governor Kor takes control of the planet, while Kirk and Spock attempt to organize resistance. The situation escalates as both fleets prepare for battle, but the Organians finally reveal that they are immensely advanced, incorporeal beings.

The Organians instantly incapacitate the Federation and Klingon forces and compel both sides to stop fighting. They predict that the two powers will eventually cooperate, foreshadowing the uneasy alliance seen in later Star Trek stories. “Errand of Mercy” first aired on March 23, 1967, and introduced Kor, who later returned in The Animated Series and Deep Space Nine. It is sometimes confused with “Day of the Dove,” another Original Series episode involving a Federation-Klingon conflict, but that story has a different alien antagonist and no Organian intervention.