The Prime Directive of Non-Interference, also known as the Prime Directive or the Non-Interference Directive, were the colloquial terms for the Federation Starfleet's General Order One. The Prime Directive dictated that no Starfleet personnel knowingly interfere with the natural progression of pre-warp civilizations.
The origins of the Prime Directive began with humanity's first great exploration of the galaxy around them in the 2150s, with the Enterprise. Although the Vulcans had developed their own non-interference directives in the 1870s, humans were not bound by these restrictions. (TOS novel: Strangers from the Sky)
In 2151, the Enterprise made contact with the Fazi and the Hipon on the planet Fazi. When Captain Jonathan Archer discovered that the Fazi were less developed than humans, he immediately wanted to begin to share technology and information with those people. However, persuasion from the Hipon, a more advanced species on the planet's southern continent, urged him not to proceed. Following the advice from Subcommander T'Pol, he decided not to share technology to allow the two races to communicate in an attempt to let the two races advance at their own rate. However, Archer decided that guidelines governing first contacts and cultural contamination were needed. (ENT novel: By the Book)
Even after the foundation of the Federation in 2161, no rules had been established to govern contact with other species, and during the 2160s there were a number of cultural contaminations of pre-warp civilizations, the most notable case being the Horizon's visit to Sigma Iotia II. While the initial contact went well, an Horizon crewman left a book, Chicago Mobs of the Twenties, behind on the planet. Over the next century, the Iotians modeled their civilization on the book. (TOS episode: "A Piece of the Action")
By the 2170s, it was decided by the Federation Council that action needed to be taken and in 2175 the Resolution of Non-Interference was drafted and signed by all Federation members, putting together a single philosophy to be adopted. By the 2190s, the Prime Directive had come into force. (TNG - Double Helix novel: Double or Nothing)
- The Last Unicorn RPG module: All Our Yesterdays: The Time Travel Sourcebook gives the Federation's adoption of the Prime Directive as occurring in the year 2220.
The philosophy of the Federation was all well and good, but a method needed to be adopted in order to measure the cultural development of a civilization. In the early 2200s, the Richter Scale of Culture became the chosen method to monitor a civilization's progress. (TOS novel: Prime Directive)
Elements within the Prime Directive prevented Starfleet officers from interfering in internal matters of other races. However, they were able to help in negotiating in a compromise so long as both sides agreed to it. (TOS video game: Judgment Rites)
Though the world of Treva lacked spaceflight, it had been contacted by and traded with non-Federation cultures prior to its contact with the Federation, and made a preliminary application for membership in 2349. As a Starfleet preliminary survey team had made a non-negative report on the world, Starfleet was able to provide appropriate aid in its political difficulties at the request of its elected government. (TNG novel: Survivors)
The Prime Directive was a Starfleet regulation, however, and not a civil law. As a result, Federation civilians were legally allowed to engage in activities that would be considered violations of the Prime Directive were they in Starfleet. In 2364, several Federation citizens on the planet Angel I attempted to influence that world's matriarchal society to embrace a more egalitarian ideal; Federation Starfleet personnel on site determined that they could not force those civilians to refrain from influencing Angel One's society. (TNG episode: "Angel One") The mercenary band of the Silver Paladin, comprising Federation citizens and former Starfleet officers, assisted in a rebellion on the planet Treva that same year. After uncovering existing alien interference, the necessary actions of active Starfleet officers, Data and Natasha Yar were waived and the USS Enterprise-D was able to assist. (TNG novel: Survivors)
Similarly, Lwaxana Troi, a Federation delegate with diplomatic immunity, was able to persuade scientist Timicin to reconsider his Kaelon commitments, a 1400-year-old absolute, in 2367. (TNG episode: "Half a Life")
In 2368, the Directive was again in question when a J'naii pilot, Soren, wanted to break tradition and choose the female gender. Although influenced by mutual affection for William T. Riker of Starfleet, s/he made a decision hirself, and ultimately accepted that hir mind be "reformed" back to one accepted by society. (TNG episode: "The Outcast")