The United Federation of Planets is one of the foundational organizations of the future galaxy in the Star Trek franchise. The Federation itself has an interesting history that goes back hundreds of years, and its mission has always been one of peace, science, and exploration.
It sounds idealistic, and that's because it is: a symbol of the civil rights movement of the 1960s that inspired most of the social-justice allegories integrated into the early TOS episodes. As the real world marches on, so does the mission and message of the Federation, and it hasn't always been one of high moral standards.
6 Founding Of The United Federation of Planets, 2161
The United Federation of Planets (UFP) was a concept that several governments with space travel-capability had been entertaining for years, but the Battle of Cheron in 2160 strengthened the concept and finally prompted the dream to become a reality. This was one of the earliest and biggest conflicts that occurred on an interplanetary level, and it involved an alliance of Earth, Andorians, Vulcans, and Tellarit versus the Romulan Star Empire.
Only through cooperation were the Romulans defeated, and the first four members of the new United Federation of Planets rose from the ashes. At its peak about 200 years later, the UFP would have more than 150 members.
5 The Return Of The Romulan Star Empire, 2266
More than one hundred years after the Romulan Empire had suffered one of its greatest defeats and retreated behind the neutral zone, they returned in a covert but aggressive way. A Romulan ship called a Bird of Prey attacked several outposts along the neutral zone in a TOS episode, "The Enterprise Incident," and it was only due to some clever espionage on the part of Kirk and Spock that an intergalactic incident was averted.
The Romulans didn't choose this period on a whim. Hostilities with the Klingons had been heating up for years, culminating in the Treaty of Organia on a year later in 2267. If not for the technologically superior Organians and their intervention, a full-blown war might have ensued.
4 Praxis Explodes, 2289
The last Star Trek movie to star the cast from TOS, The Undiscovered Country, tells the story of this crucial incident in depth. The plot kicks off with a brutal explosion on the moon of Praxis, which the Klingons use as a fuel resource. The moon is not only destroyed, but will render the atmosphere of the Klingon homeworld uninhabitable. Aid was required to evacuate the planet and dismantle any physical symbols of Klingon aggression throughout the galaxy.
It was a time of tension, to put it mildly. The original arbiter of the plan, Klingon Chancellor Gorkon, was assassinated after an initial meeting with a Federation delegation, headed by Starfleet personnel including Admiral James Kirk and Commander Spock. It was only due to the efforts of the Enterprise crew that a second assassination attempt was averted and a plan to integrate the Klingons into the Federation went forward.
3 The Tomed Incident, 2311
Based on a novel set during the TOS era, the Tomed Incident was one of the biggest false flag operations in galactic history. The original story was simply that a Romulan warship, the Tomed, had crashed into a Federation base at warp speed.
The resulting explosion not only destroyed the base but the USS Agamemnon and 13 other federation outposts. It turned out that the whole attack has been staged by Starfleet Intelligence to prompt the Romulans to stand down and encourage the Klingons to align more closely with the Federation.
2 The Treaty of Alliance, 2353
More than 50 years after the destruction of Praxis, the agreement also known as the Klingon Alliance was signed between the Klingon Empire and the United Federation of Planets. This brought an end to centuries of tension between two of the galaxy's most powerful political forces, ensuring an era of peace in both the Alpha and Beta quadrants of the galaxy and pushing the Romulan Star Empire to the proverbial sidelines.
This was a controversial deal that had its share of detractors and enthusiasts. On one hand, it entitled the Klingons to request Federation and also Starfleet aid for virtually anything, but on the other, it finally allowed for free travel and trade between the two cultures at an unprecedented level.
1 Contact With The Borg, 2365
The Borg was one of the reasons that Star Trek: The Next Generation became one of the most popular shows on television. These terrifying villains weren't going to respond to the Federation's message of understanding and the free exchange of ideas, and new solutions were needed to deal with this threat.
It was the heroes of The Next Generation that were first tested by the Borg, with Captain Picard assimilated at one point. Less than a generation later, Captain Janeway had to navigate Voyageur through Borg space to get her crew back home. Ironically, it was Borg technology that allowed Janeway to bring her crew home a few years early, and the knowledge her ship brought home has been instrumental in helping the Federation build a force to resist the Borg.