Originally meant to be just a one-off species of the Star Trek franchise, the Breen were first mentioned in the fourth season of Star Trek: The Next Generation in 1990. Even though they were referenced in several other episodes from then on, they did not actually appear on-screen until the episode "Indiscretion" of Star Trek: Deep Space Nine in 1996.
The writers admitted that they had previously used the name "Breen" whenever they needed a random alien species for a storyline. Hence, not much was known about them up until much later on. So, who exactly are the Breen, and where do they fit into the expansive universe of Star Trek?
What Do the Breen Look Like?
There is a lot of speculation on what these bloodless creatures actually look like, as the Breen are always seen in environmental suits that cover them from head to toe. It is suggested that this is because their home planet experiences extremely low temperatures. During the Dominion War, the Dominion and Cardassian leaders receive intelligence reports that describe the Breen planet as "a frozen wasteland." However, the Vorta field supervisor Weyoun describes it "quite comfortable" later on.
The Breen’s suits feature metal armor and a detachable and 'vented snout,' giving the impression they are a snouted species. The top of the helmet has a visor that either glows green, or has green and red lights on it. The suits are of two types: a standard suit that most of the Breen wear, and a more elaborate suit worn by Thots, the authority figures of the Breen Confederacy. (The Thots are comparable to the Legate of the Cardassian military in terms of rank, according to the Cardassian leader Damar.) With distinct gold lines running across the helmet, this decorative suit reinforces the elevated status of the Breen leaders.
They are also more resilient than other humanoids. For example, when a Breen is struck by the Klingon warrior Worf, the Breen seems to be completely unaffected by the attack and proceeds to assault him instead. The Klingons have famously coined the phrase: "No one has ever seen [a Breen] and lived to speak of it." This is partially in reference to the fact that Breen are extremely territorial; a fleet of Klingon warships had been sent to conquer their home world during the Klingon Second Empire, which never returned and was never heard from again.
The Breen also have an unusual and incomprehensible speech, further adding to their mysterious image. The scratchy, electronic warbles cannot even be deciphered by the universal translator, which is not the case with most of the other species in the franchise. Empathic species such as Betazoids are also unable to sense the Breen’s feelings and emotions, much like the Ferengi.
What Role Did the Breen Play in the Dominion War?
The Dominion, a planetary alliance in the Gamma Quadrant, came into conflict with the Alpha Quadrant powers, ultimately leading to the Dominion War seen throughout much of the Deep Space 9 series. Although the Breen have never been inclined to make political alliances, the writers felt that their involvement could heighten the stakes of the war. Thus, talks with the Female Changeling, the head of the Dominion, led to Breen leader Thot Gor agreeing to provide reinforcements in 2375. The Dominion were interested in the Breen’s military weapons, and made a deal with them: they would hand over the control of several Cardassian and Federation planets once the war was won. However, it is later implied that this was a false promise.
To honor their partnership, the Breen ‘gifted’ Worf and the Deep Space 9 counselor Ezri Dax to the Dominion. They went on to win the Second Battle of Chin'toka, after destroying most of the ships that belonged to the Federation and causing major damage to Earth. A highly advanced weapon called the energy dissipator, which can drain a starship’s power systems, enable the Breen’s victory. Only the Klingon’s ship turned out to be immune to the device, while the rest were defeated.
The growing relations between the Breen and the Dominion caused the Cardassian Union, another major ally, to become bitter. Cardassian leader Damar was not pleased with the terms of the alliance between the two parties, which gave the Breen full access to the Dominion’s database — which wasn't permitted to the Cardassians. This turned out to be one of the major reasons why Damar eventually chose to switch to the other side and lead a rebellion to free Cardassia.
This major development, combined with the fact that the Federation have created a defense against the energy-dampening weapon, led to power shifts in the war. Thot Pran, who succeeded Thot Gor as military commander, asked the Female Changeling for confirmation that the Dominion were not surrendering, but strategizing for another attack instead. This, once again, showcases how the Breen are ruthless fighters. The Romulans, much like the Klingons, have a saying regarding this race: "Never turn your back on a Breen." This is demonstrated when a captive Breen takes a disruptor from the holster of a guard when his back is turned, and uses it to kill two guards at once.
In the Battle of Cardassia, the Breen fought on the front lines, even ahead of the tough and devoted Jem'Hadar forces of the Dominion. They proved to be a tough adversary for the Federation once again. Even though they lost the war, the story arc could not have been as compelling without their formidable strength and their willingness to die for the cause till the very end.
As allies of the Dominion, the Breen are present at the signing of the Treaty of Bajor. Though their status after the war has not been discussed in any later media, the treaty has no implications regarding the Breen, seemingly leaving them in the clear.