Star Trek's United Federation of Planets has many allies who share the same values of peace, cooperation, and mutual prosperity. There are those in the Star Trek universe though, who do not share these ideals. The Alpha, Beta, Gamma, and Delta Quadrants in both the Prime and Mirror Universe are rife with Empires that are built upon war, domination, and conquest, and not on inclusivity and diversity as the Federation is.
Some of the more violent Empires have also conquered others, and some occupy vast regions of space owing to their ruthless pursuit of power. These are the supreme Empires in Star Trek, ranked not only by their size but also by their determination to rule the galaxy.
7 The Breen Confederacy
The Breen are one of the big, yet mysterious, antagonists from Star Trek: Deep Space Nine. They also make a surprise appearance in Season 3 of Star Trek: Lower Decks. Forging alliances with all sorts of nefarious Empires, such as the Cardassian Union, the Dominion, and the Ferengi, the Breen are one of the most underestimated powers in the Alpha Quadrant.
At some point in Star Trek canon, the Breen rule over a relatively large part of the galaxy and occupy three planets. They are, however, defeated by the Federation and its allies in the Battle of Cardassia in 2375, which is the final battle of the Dominion War.
6 The Cardassian Union
Mostly explored in episodes from Deep Space Nine, the Cardassian Union is one of the most powerful empires in the Alpha Quadrant, until its collapse after the Dominion War in 2375. Although the Cardassian Union does not span as many parsecs as other powerful Empires, according to canon, there are around 40 planets that belong to the Union.
Led by the infamous Cardassian, Gul Dukat, the Cardassian Union became a totalitarian militaristic regime. Although the Federation had a peace treaty with the Cardassians, other rebellious groups like the Maquis resisted its aggression.
5 The Romulan Star Empire
The Romulans are a group of Vulcans who left Vulcan to settle on the planets Romulus and Remus, as they did not agree with Surak's philosophy of peace. Spanning over the Alpha and Beta Quadrants, the Romulan Star Empire reigned until the late twenty-third century.
The Romulan Star Empire is characterized by extreme xenophobia and secrecy, all of which are explored in many different Star Trek series, such as Enterprise, Deep Space Nine, and The Next Generation. The Romulan Star Empire might not have as many member planets as the Cardassian Union, but it certainly spans a larger area of space and became very powerful owing to its subterfuge.
4 The Dominion
Many Star Trek fans would agree that the Changelings are some of the most terrifying villains in the franchise. In order to shield themselves from persecution by what are considered the "solids," the Changelings established the Dominion.
The Dominion soon became the rulers of the Gamma Quadrant, partly owing to genetic engineering, and invaded the Alpha Quadrant upon discovering the wormhole next to Deep Space 9. Owing to its domination of many planets, and affiliations with species that included the Cardassians and the Breen, the Dominion became one of the most feared Empires by the Federation, until its surrender in 2375.
3 The Klingon Empire
The Klingons are likely one of the most famous races in Star Trek, but also one of the most bloodthirsty and violent. The Klingon Empire rules over almost thirty planets and inhabits a large part of the Beta Quadrant. There are many wars fought between the Federation and the Klingons throughout many series of Star Trek, however, these two opposing sides do join forces later on.
Owing to their fierce fighting mentality and skill in battle, the Klingons are an Empire to be reckoned with, and they played a crucial role in defeating the powerful Dominion in 2375.
2 The Terran Empire
Even more powerful are the Terrans, who, in the Mirror Universe, were able to subdue the invincible Klingon Empire by violently destroying their home world, Qo'noS. The Terran Empire is a militaristic, totalitarian regime of humans that embodies the opposite of everything the Federation stands for.
Two known rulers of the Terran Empire are Hoshi Sato, seen in Star Trek: Enterprise, and Phillipa Georgiou from Star Trek: Discovery. Both women are relentless in their treatment and torture of races they deem unworthy of the Terran Empire. Although the Terran Empire exists only in the Mirror Universe, it has managed to subjugate powerful races, such as the Klingons, Vulcans, Andorians, Orions, and many others.
1 The Krenim Imperium
Although the Krenim only feature in two Star Trek episodes - the thrilling Voyager two-parter, "Year of Hell" - they are a formidable Empire in the timeline where Captain Janeway does not destroy their time-ship. At its height, the Krenim Imperium spans over five thousand parsecs and controls over two hundred star systems and nine hundred planets.
The Krenim manage to subdue a large portion of the Delta Quadrant with their unique temporal weapons. The military leader and scientist, Annorax, invents a ship that can erase entire planets from the space-time continuum, leading to the destruction of hundreds of civilizations - until Captain Janeway resets the timeline by crashing Voyager into their time-ship of course.