Throughout Star Trek's long history, the United Federation of Planets has faced terrifying and powerful enemies. Captain James T. Kirk (William Shatner) and his USS Enterprise crew encountered numerous alien enemies throughout their voyages, but some proved to be more malevolent than others. While many villains from Star Trek: The Original Series only appeared in one episode, others, like the Klingons and the Romulans, became staples of the franchise. Although the Federation managed to achieve peace with some former enemies, others continue to threaten humanity's way of life.

Whether under the command of Captain Kirk, Captain Jean-Luc Picard (Patrick Stewart) or others, every version of the Starship Enterprise often ends up on the front lines, facing the Federation's most formidable enemies. Sometimes this leaves the Enterprise and its crew as the only thing standing in the way of a devastating Federation takeover. The USS Discovery faces a similar problem in the far future of Star Trek: Discovery, as its spore drive propulsion system making it one of the Federation's most important ships. Whether they had a grudge or simply wanted what the Federation had, these enemies were powerful enough to threaten the entire United Federation of Planets.

10 The Emerald Chain

Star Trek: Discovery

The Emerald Chain developed sometime in the late 31st century after the Burn devastated Starfleet and the Federation. A capitalist syndicate of Orions and Andorians, the Emerald Chain was led by an Orion woman named Osyraa (Janet Kidder). In the power vacuum caused by the Burn, the Chain rapidly gained supremacy in many parts of the galaxy, initiating first s without following any of the Federation's protocols, and taking over numerous planets.

The Emerald Chain posed a very real threat to the Federation.

By 3189, over fifty star systems had been overtaken by the Emerald Chain, and the Federation did not have enough ships to intercede. Minister Osyraa held the USS Discovery's crew hostage, forcing iral Charles Vance (Oded Fehr) to meet her for negotiations. Vance ultimately rejected her offer, and the Emerald Chain later splintered after Osyraa's death. In normal circumstances, the Federation would have been able to easily combat the Emerald Chain, but with so many of Starfleet's ships destroyed in the Burn, the Emerald Chain posed a very real threat to the Federation.

9 Species 10-C & The Dark Matter Anomlay

Star Trek: Discovery

In Star Trek: Discovery season 4, the Federation faced a galactic threat in the form of the Dark Matter Anomaly, or DMA. The DMA appeared in 3190 as a massive gravity well that consumed everything in its path. Starfleet and the USS Discovery began studying the DMA but were unable to prevent it from destroying Kwejian and its moon. The creators of the DMA became known as Species 10-C and Discovery set out to initiate first with the strange species beyond the Galactic Barrier.

First with Species 10-C marked the first time the USS Discovery traveled to another galaxy.

Captain Michael Burnham (Sonequa Martin-Green) and her crew discovered that Species 10-C used the DMA to mine boronite and were completely unaware of the destruction they were causing. After Discovery's crew found a way to communicate with Species 10-C, the aliens agreed to stop using constructs like the Dark Matter Anomaly. Although the conflict with Species 10-C turned out to be a massive misunderstanding, the DMA destroyed an entire planet and its civilization, making them formidable, albeit temporary, enemies of the Federation.

8 The Breen

Star Trek: Deep Space Nine, Star Trek: Discovery

Not much was known about the Breen in Star Trek's 24th century, but they were warlike and extremely aggressive toward anyone who entered their territory. The Breen allied with the Dominion during the Dominion War and launched one of the most devastating attacks against Earth, causing massive damage to San Francisco and Starfleet Headquarters. The Breen continued to deal blows against the Federation during the Dominion War, and they remained enemies of the Federation for centuries to come.

In an alternate timeline glimpsed in Star Trek: Discovery season 5, episode 4, "Face the Strange, the Breen had acquired the Progenitors' technology and destroyed the Federation.

Although the different factions of the Breen Imperium were involved in a civil war in the 32nd century, they remained formidable Federation enemies. When the USS Discovery sought the Progenitors' technology in Star Trek: Discovery season 5, the Breen reared their gelatinous heads yet again. Breen Primarch Ruhn (Tony Nappo) sought to apprehend his nephew, L'ak (Elias Toufexis), and he sent a massive Breen Dreadnought to Starfleet Headquarters. The Federation managed to avoid a war, but Discovery later clashed with the Breen at the Eternal Gallery and Archive.

7 The Vau N'Akat

Star Trek: Prodigy

When the Federation intially made first with the Vau N'Akat on Solum in Star Trek: Prodigy, the planet's people disagreed about whether or not to embrace other species. This disagreement led to a devastating civil war that would have led to the Vau N'Akat's extinction. Fifty years after First , the USS Protostar emerged above the planet. The surviving Vau N'Akat developed a plan to send the Protostar back into the past with a weapon on board that would destroy the Federation and prevent First from ever happening.

The cast of Star Trek: Prodigy in front of the USS Protostar
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When Captain Chakotay (Robert Beltran) sent the Protostar through the wormhole early, it was lost in time and space, where it would eventually be found by Dal R'El (Brett Gray). Not only did the Vau N'Akat plot to destroy the Federation with the Protostar, but Asencia (Jameela Jamil) also conspired with modern Solum to attack the Federation. Using temporal technology stolen from the mind of The Traveler Wesley Crusher (Wil Wheaton), Asencia prepared to launch a devastating attack, but her plans were thwarted by Dal and his friends.

6 The Loom

Star Trek: Prodigy

In Star Trek: Prodigy season 2, the Loom threatened not only the Federation, but Star Trek's entire multiverse. A trans-dimensional species fueled by paradoxes, the Loom emerged in the Prime Star Trek timeline after Dal R'El (Brett Gray) and the young crew of the USS Protostar inadvertently changed history. The Loom also pursued Wesley Crusher as he attempted to restore the timeline, and even with his vast Traveler powers, he barely managed to escape.

The Loom advances on Maj'el in the temple in Star Trek Prodigy
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The Loom can destroy entire timelines, completely erasing them from existence. They can manipulate time, slowing it to a crawl in order to consume their prey, and most phaser and shields have no effect on them. Wesley helps hold off the Loom long enough for Dal and his friends to restore the proper timeline, sending the Loom away to seek out a different paradox. The Loom looked amazing rendered in Star Trek: Prodigy's dynamic animation and they will remain a threat any time someone messes too much with the timeline.

5 The Klingons

Star Trek: The Original Series, Star Trek: The Next Generation, Star Trek: Deep Space Nine, Star Trek: Discovery

The Klingons made their debut in Star Trek: The Original Series season 1, episode 27, "Errand of Mercy," and have had a tumultuous relationship with the Federation ever since. The Federation was engaged in a cold war with the Klingons in the early 23rd century, before true war broke out at the Battle of the Binary Stars in 2256. Star Trek: Discovery season 1's Klingon War engulfed the galaxy as the Klingons dealt the Federation massive damage.

With the USS Discovery stranded in the Mirror Universe, the Federation lost one-third of its ships and twenty percent of its territory to the Klingon Empire. Had Discovery emerged from the Mirror Universe much later, the war may have been lost. The war ended in 2257, when Michael Burnham helped L'Rell (Mary Chieffo) claim control of the Klingon Empire. This was followed by alternating periods of uneasy peace and violent conflicts, until the Khitomer Accords were signed in 2293 in Star Trek VI: The Undiscovered Country.

4 Q

Star Trek: The Next Generation, Star Trek: Picard

John de Lancie's Q may have developed a particular fondness for Captain Jean-Luc Picard on Star Trek: The Next Generation, but that did not mean he was above threatening the entire Federation. In their very first meeting in TNG's series premiere, "Encounter at Farpoint," Q put Captain Picard and his crew on trial to answer for the crimes of humanity, and he has been a thorn in the Federation's side ever since.

Although Q could occasionally be helpful, his motivations were never quite clear, and he often caused more harm than good.

For example, Q introduced the Federation to the Borg, which allowed Starfleet to prepare, but also made the Borg aware of the Federation earlier than they otherwise would have been. In Star Trek: Picard season 2, Q altered the course of human history, transforming the utopian Federation into a xenophobic totalitarian regime known as the Confederation of Earth, just to "help" Captain Picard face his childhood trauma.

3 The Romulans

Star Trek:: The Original Series, Star Trek: The Next Generation, Star Trek: Enterprise, Star Trek: Picard

Introduced on Star Trek: The Original Series, the Romulan Star Empire have long been one of the Federation's most cunning and duplicitous enemies. Distant cousins of the Vulcans, the Romulans were xenophobic and sought to conquer other worlds rather than ally with them. As Earth was becoming a major power in the Alpha Quadrant, an alliance formed between the humans, Vulcans, Tellarites, and Andorians. These four worlds fought the Romulans during the intergalactic Earth-Romulan War from 2156 to 2160.

Star Trek: Picard revealed Romulans not only infiltrated Starfleet, but the Mars Attack of 2385 forced the Federation to rescind into its borders and ban synthetic life forms.

Although the Coalition of Planets ultimately triumphed, the Romulans remained enemies of the Federation for centuries. Tensions were still high in the late 24th century when the Romulan sun went supernova. Although Starfleet initially aided in the evacuation of the Romulan people, these efforts were halted after a devastating attack on Mars that left almost 100,000 dead. The Romulan cabal known as the Zhat Vash were the orchestrators of the Mars attack, cementing the Romulans as one of the Federation's most dangerous enemies.

2 The Dominion

Star Trek: Deep Space Nine

Ruled by a group of Changelings known as the Founders, the Dominion proved to be one of the most formidable opponents the Federation had ever faced. After the Bajoran wormhole was discoverd in Star Trek: Deep Space Nine's premiere, Starfleet began hearing rumblings about the Dominion. With their need to dominate other cultures, the Changelings infiltrated the Alpha Quadrant, slowly destabilizing some of its major powers. The full-scale Dominion War broke out in 2373, as the Dominion and their Cardassian allies took control of space station Deep Space Nine.

A composite image of Captain Sisko and Weyoun from Star Trek Deep Space Nine
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Starfleet and the Federation suffered a number of devastating defeats and by the end of 2374, the Dominion was poised to win the war. Captain Benjamin Sisko (Avery Brooks) deceived the Romulans to get them to the fight and the Cardassians began rebelling against Dominion forces. Still, the Dominion-Breen alliance dealt the Federation numerous devastating blows, including an attack on Starfleet Headquarters. After Section 31 infected the Changelings with a lethal virus, Odo (René Auberjonois) eventually convinced the female Changeling (Salome Jens) to surrender, but the costs to the Federation, the Klingon Empire, and the Cardassian Union were great.

1 The Borg

Star Trek: The Next Generation, Star Trek: Voyager, Star Trek: Picard

Since their introduction on Star Trek: The Next Generation, the Borg have become one of the most formidable enemies of the Federation. From their very first appearance in TNG season 2, episode 16, "Q Who," the Borg were unlike any enemy the Federation had ever faced. They could not be reasoned with, and they adapted quickly to new weapons and technology. The Federation has been involved in several conflicts with the Borg, including the devastating Battle of Wolf 359.

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After being assimilated and turned into Locutus of Borg, Captain Picard faced the Borg again in Star Trek: First , preventing them from altering the timeline. In Star Trek: Picard season 3, Picard and his USS Enterprise-D crew saved the Federation from the Borg yet again, thwarting a plot orchestrated by the Borg Queen (Alice Krige) and the Changelings. Although most of Star Trek's Borg have been destroyed, the damage they caused Starfleet and the Federation will not be fading from anyone's mind anytime soon.