Inhabitants of Ekos, whose society was reshaped by Federation historian John Gill into a copy of Nazi Germany.
The Ekosians are humanoids from Ekos, a world sharing a star system with the more advanced planet Zeon. Federation historian John Gill, sent as an observer, tried to bring order to the fractious Ekosians by imposing a government modelled on Nazi Germany, believing he could keep its efficiency without its brutality.
Kirk and Spock found that the experiment had gone terribly wrong. Gill had been drugged into a puppet by his deputy Melakon, who used the regime to persecute the Zeons. With help from the Ekosian underground, the landing party exposed Melakon and the regime began to dismantle itself.