The Federation historian John Gill visited Ekos in the mid 23rd century. What Gill found was a planet in the throes of anarchy. Gill chose to violate the Prime Directive in order to help the native population. Gill tried to institute a modified version of a Nazi government on Ekos - believing that a benign Nazi government that did not include the worst excesses of the original party could bring order to the Ekosian people without the disastrous consequences the original had to not only the German people, but to millions of others as well.
What Gill didn't realize was that the structure of governments structured on that of Nazi Germany during the 1930s and 1940s inherently invited abuse. Gill was drugged by his subordinate Melakon, who then became the real power on Ekos. Once Melakon took over, the worst aspects of Nazi rule came to the forefront on Ekos. The people of Zeon became the targets of this Nazi government, much like the Jewish people had been the targets of the original party.
Captain James T. Kirk and the USS Enterprise arrived to find out what had happened to Gill, and discovered that his tampering in the Ekosian government had led to the rise of the Nazi state on Ekos. They found the drugged Gill, and revived him in time to put a stop to Melakon's plans. Melakon shot Gill, and was in turn killed by another Ekosian. Afterwards, a resistance movement took control of the Ekosian government. As Gill died in Captain Kirk's arms, he admitted that he made a terrible mistake, and that the Prime Directive was the correct way to deal with younger cultures. (TOS episode: "Patterns of Force").
Though Kirk hoped the collapse of the Ekosian government would lead to peace, instead both Ekos and Zeon became more xenophobic and aggressive. Then, at some point between 2268 and 2287, the Klingon Empire initiated contact with the M43 Alpha system, and began providing arms to both sides. This led to an arms race which exploded into full-scale conflict in 2287. The conflict continued even after the Klingons withdrew following the explosion of Praxis in 2293. The Federation attempted to broker a peace between Ekos and Zeon, but as of stardate 34675.8, the conflict continued, and had claimed the life of Starfleet Lieutenant Shewer Freeman. At that time, Commodore Göller recommended withdrawing all Federation personnel from the system. (SCE eBook: Fables of the Prime Directive)