Every science fiction show set in space needs one or more alien species. As Star Trek proved, not all the aliens the main characters encounter are the good guys. Of all the Star Trek antagonists, the Borg easily belong among the most frightening ones. Not only do they work in large numbers and are technologically advanced, but they also have no emotions and as such no mercy toward those they want to assimilate.
The Borg collective strips away everything that makes the victim unique and leaves only a Borg drone. Considering the fact that Star Trek is told primarily from the Starfleet's perspective, almost all the noted important moments in the Borg's history had something to do with the Starfleet and humans in one way or another.
5 The Origin Of The Borg
Every species has to start somewhere, and the Borg are no exception. While there isn't as much information about their origin as with other species, Star Trek provided several interesting details over the years. The Borg come from the Delta quadrant, and it took them thousands of years to evolve into a life form that combined organic matter with technological implements.
The Borg started their wider assimilation process in the 15th century and by the time most Star Trek events take place, they have already successfully assimilated thousands of systems and species, as well as absorbing their technological advancements. That helps to understand why the Borg are so powerful - unlike the Federation that existed for only two hundred years in the 24th century, the Borg had almost a millennium to improve their Collective and to make it as close to impenetrable as possible.
4 The Borgs Fail To Rewrite Human History
Every Star Trek fan knows that the 5th of April 2063 is the day Vulcans made the first contact with humans, which changed Earth's history forever. However, this joyous occasion (even though some anti-alien humans might not agree with such an assessment) has another side to it. It's also the moment of one of Borg's biggest failures. The Borg traveled back in time and initially managed to stop the first contact and change all of humanity, assimilate everyone.
Captain Picard and his crew went back in time as well and helped Zefram Cochrane, the inventor of the warp engine, to fix his ship so that the first contact would happen like it was supposed to. At the same time, the Enterprise's crew led a dangerous battle with the Borg onboard the ship, which wasn't without human casualties. The most memorable moment is when Captain Picard and Lieutenant Data take on the Borg queen... and win. It was a good day for the Starfleet, but not so much for the Borg who never attempted to pull off something like this again.
3 The Borg Encounter Captain Picard's Enterprise For The First Time
John de Lancie's Q is one of the most prominent and popular antiheroes of the Star Trek series. He helped the heroes sometimes but brought them into trouble other times. In 2365, in order to prove to Captain Picard and his crew that they're not ready for every danger the universe has to offer, Q brings the ship right into the path of the Borg.
The Borg take interest in the technologically advanced human ship and from there on, make more effort to assimilate humans than ever before. The Borg have assimilated humans in the previous years, such as Annika Hansen aka the Seven of Nine, but it isn't until this very event that the Borg truly turn their sight toward the Starfleet and humans.
2 The Borg Assimilate Jean-Luc Picard
The assimilation of Captain Picard happened in 2367 and even though it originally seemed like an everyday occurrence to the Borg, it would blow up in their faces in the end. Not only was the Enterprise's crew able to free Picard from the Borg, but the remaining link between Picard and the Borg allowed him to hinder their plans in the future.
For example, this happened when Captain Picard used the connection in 2373 to discover a weakness in a Borg cube's shield matrix and subsequently told the information to the rest of the fleet that was protecting Earth from the Borg attack. This means that by assimilating Picard a few years earlier, the Borg created a tactical disadvantage for themselves.
1 Voyager Defeats The Borg
Captain Picard's Enterprise wasn't the only Starfleet ship that was able to deal a fatal blow to the Borg collective. The starship Voyager, captained by Kathryn Janeway, managed to do the same. Voyager was forced to deal with the Borg more often than almost any other Starfleet ship after they had ended up in the Delta Quadrant.
They not only saved Seven of Nine from the Borg collective but also defeated them in 2378. The crew used a future technology to use and destroy the transwarp hub, and it released a neurolytic pathogen among the collective, doing them a lot of damage. Overall, this occasion was another significant failure in the Borg's history that made it obvious the Borg aren't undefeatable despite their technological advancements.