In a first season episode of Star Trek: The Next Generation the Enterprise visits a planet named Rubicun III where (according to Memory Alpha) the Edo live.
It is often mentioned in Star Trek that the Prime Directive requires that pre-Warp civilisations be ignored until they discover Warp mechanics. But this planet and civilisation appears to be exempt from that status. Why?
In the case of the Next Generation episode Justice, in which the Edo appear Wikipedia indicates:
In contrast, the Next Generation episode "Justice" did not explicitly explain whether the Edo people were pre-warp or were aware of offworld space travelers prior to the Enterprise's visit. If the case is the former, then when Wesley Crusher is sentenced to death, the violation of the Prime Directive had already occurred and the issue of rescuing him, while politically exacerbating matters, might not have been a violation of the Directive.
Emphasis is mine.
Violations of the Prime Directive are numerous and recounted in the Prime Directive entry in Wikipedia:
One criticism regarding the Prime Directive is that it is inconsistently applied, depending on a planet's strategic importance or the circumstances in which a starship crew finds itself. For example, as part of the Federation's then-ongoing hostilities with the Klingons, Captain Kirk was ordered to make contact with the seemingly pre-industrial Organians in "Errand of Mercy." In addition, Kirk directly interfered with the laws or customs of alien worlds in "Friday's Child," "For the World Is Hollow and I Have Touched the Sky," "The Cloud Minders," "The Apple," "The Return of the Archons," and "A Taste of Armageddon," in order to achieve a Federation objective, to save the lives of his crew, or both.
Compounding matters is that in the TOS episode "The Omega Glory," Kirk states, "A star captain's most solemn oath is that he will give his life, even his entire crew, rather than violate the Prime Directive," and yet he seemingly violates the Prime Directive as "the only way to save my ship" in "A Taste of Armageddon" and no explanation for the Federation Ambassador trying to mediate between Eminiar VII and Vendikar (neither of which are Federation members) regardless of their wishes on the matter is given.
In "The Return of the Archons" and "The Apple" reference to the "Prime Directive of non-interference" is made by Spock. In "The Return Of The Archons," Kirk says the Prime Directive refers to "a living, growing culture" to justify interfering with what he sees as the non-development of the computer-controlled culture, asking pointedly in reference to it, by contrasting it with living, growing cultures, "Do you think this one is?" In the "The Apple" Spock points out that Starfleet Command may not agree with his choice to interfere with the computer controlled culture to which Kirk replies "I'll take my chances."
There are also episodes in which the Prime Directive should have been mentioned, but wasn't. In "The Paradise Syndrome"," the Enterprise attempts to save a pre-industrial planet by moving an asteroid that was on a collision course with it; when McCoy asks Kirk if he should warn the people, Kirk and Spock only point out the people would not understand the warning, and neither makes any reference to the Prime Directive. In "The Cloud Minders Kirk interferes with the culture of Ardana to obtain zenite the only cure for a biological plague ravaging Merak.
Admiral Matthew Dougherty's reasons for violation of the Prime Directive in Star Trek: Insurrection in Picard's time echoes the reasons Kirk gives McCoy in "Private Little War" but Picard considers them invalid. In "Homeward," Nikolai Rozhenko (Paul Sorvino) uses holodeck technology to save the Boraalan and enforce what he believes is the spirit of the Prime Directive even though Picard has already said such actions violate what it actually states. In "Pen Pals," Captain Picard rectifies contact with an inhabitant of a pre-warp planet by ordering her memory erased. When contamination became too serious to be fixed by memory erasures, Captain Picard decided to make direct contact with a civilization's leaders in "Who Watches the Watchers" and "First Contact," although the latter episode involved a planet on the verge of achieving warp flight, and therefore eligible for First Contact. Finally, in "Natural Law," the Voyager crew took measures to ensure the protected isolation of a primitive people, even from a more advanced civilization who share the same planet.
The Prime Directive rules against revealing the presence of extra-terrestrials only seems to apply to pre-contact civilisations, not pre-warp civilisations. On numerous occasions (TNG: Code of Honor, TNG: Justice, TNG: Pen Pals, TOS: Errand of Mercy, etc) it's made abundantly clear that as long as a planet is already aware of the existence of extra-terrestrials, it's OK for Starfleet to have diplomatic relations with them. At that point, the Prime Directive appears to switch to one of preventing undue influence.
In this particular case, Rubicun is described (by Data) as being part of a star-cluster of ...
...three thousand four other planets
Many of which are evidently suitable for both life and colonisation.
Despite their simplistic pre-warp lifestyle, it seems at least reasonable to assume that in such a densely packed environment, Edo scientists would have been aware of other civilisations through simple observation.
It seems to depend on what type of contact another civilization has had, not how advanced they are-the Federation has clearly had contact with pre-warp civilizations that already knew about "aliens."
It's interesting to note that violations by Picard and his crew are mostly accidental, and Picard does everything he can to keep things the way they are on the planet involved until they are deemed ready for contact. Kirk, on the other hand, seems more interested in deliberately changing a planet's culture ahead of time if he thinks it's necessary.
That's because it was a first season episode. It wasn't until the third season that they decided the Prime Directive was a non-contact directive. There were several open contacts with pre-warp drive cultures in the first season like Angel One and Code of Honor. It was only with Who Watches The Watchers that the rules had changed and suddenly the Federation was only observing them in secrecy.
In TOS, there appears to be an exception in place for planets in the "disputed territory" with the Klingons. This makes sense since the Klingons clearly have no such law and contamination seems inevitable. Note how "A Private Little War" referred to Kirk's previous recommendation, not the Prime Directive.
There's nothing in TOS to indicate the PD includes saving worlds from natural disasters and "The Paradise Syndrome" seems to make the opposite argument. That application of the PD started with "Pen Pals".
