Very little is know about Breen physiology as few to none have ever been recorded outside of their temperature controlled suites.
Little is know about the Breen culture. A very secretive and combative species with the Federation, little progress has been made in unlocking the secrets to their culture.
Brief Canon History
The Breen homeworld is located in the alpha quadrant. It is widely believed to be an extremely cold & inhospitable world. The Breen themselves are extremely isolationist and can be very aggressive in their rare encounters with other species. The Klingons were amongst the first species to discover the Breen and, during the second empire, Chancellor Mow’ga ordered an entire fleet to invade and conquer the Breen homeworld. That fleet were never heard from again.
By the 24th century, Breen privateer raids became increasingly common, and they were known to take prisoners to use as slave labour.
In 2375, the Breen allied themselves with The Dominion, the boldest political move in their history, and went from a state of hostility with the majority of the alpha quadrant to one of open warfare. The early days of the alliance saw the Breen inflict heavy casualties on the Federation Alliance, even attacking Earth itself, causing significant damage to Starfleet Command. The Breen were able to put the Dominion at a significant advantage over the Federation Alliance, primarily due to their unique energy dampening weapons, and they effectively supplanted the Cardassians as the Dominion’s primary allies in the alpha quadrant.
The Federation alliance was able to develop a countermeasure to the Breen energy dampening weapons, putting them back on a more level playing field. The alliance launched a counterattack, during which the Cardassians switched sides in an effort to liberate themselves from what they now saw as an oppressive regime. During the battle of Cardassia, under orders from the Dominion, the Breen turned their weapons on Cardassian cities, killing millions.
The aftermath of the Battle of Cardassia saw the Dominion & the Breen stand down, and the treaty of Bajor effectively brought the war to an end.
Proposed Post-War History
At the conclusion of the Dominion war, the Breen returned to their own space, reverting to their traditional isolationist role. The Confederacy was extremely quiet for the next few years. They had suffered considerable losses in the war, but Starfleet intelligence (SI) believed that they still held a significant fleet in reserve.
Starfleet has never been able to get an intelligence operative inside the Confederacy, nor have they been able to successfully turn a single Breen to their cause, so the Breen remain a mystery in terms of their long term goals and true military strength. SI analysts studying long range sensor scan, fleet movements and energy readings believe that the most war period was one of intensive rebuilding within the confederacy, as well as the consolidation of their core philosophies after what they considered to be a defeat.
In 2381, the Breen became a founder member of the Typhon Pact after intensive secret negotiations. he pact consisted of the Romulan Star Empire, Tzenkethi Coalition, Breen Confederacy, Gorn Hegemony, Tholian Assembly and the Holy Order of the Kinshaya, all of which had hostile relations with the Federation and/or Klingon Empire in their previous history and all of which had typically previously maintained xenophobic and isolationist policies.SI operatives within some of the other signatories reported that the Breen remained aloof throughout the negotiations, even by the group’s standards, and their motives remained mysterious. Analysts have suggested that the Breen saw the Pact as a way to ensure security of their borders without committing too many of their own ships.
The next few years saw the Breen conduct limited expansions into neutral territory, whilst supported other Typhon Pact powers. SI analysts believed that there was an overall plan for the Pact which they were building towards, although it was unclear as to whether it was to include open warfare with the Federation.
In 2387, Romulus was destroyed by the Hobus supernova. The ensuing fallout threw the Typhon Pact into chaos, and whatever plans they were working towards were abandoned. The Romulans had previously been one of the primary forces in keeping the Pact working together, and as they were no longer able to do so, the other signatories fell gradually into disarray. By 2395, the collapse of the Typhon Pact appeared imminent.
SI believes that the Breen Confederacy has all but severed ties with most of the other members of the Typhon Pact, abandoning their erstwhile allies. However, it also appears that their time working with the pact has allowed the Confederacy to rebuild their forces to pre-Dominion War levels, with some analysts believing that they may now be in possession of an even larger military than ever before.
Recent activity within the Confederacy led Starfleet Intelligence to believe that the Breen were planning a large scale expansion into unexplored space, putting them perilously close to the Federation’s outermost frontier. Starfleet made construction of a new station on this frontier, with the new outpost being named Deep Space 12 and commanded by Admiral Eldon Voorhies. A number of Starships were based out of DS12, with the defensive forces being led by the USS Tower Bridge under the command of Captain Adam Gamble. The exploratory group was initially headed up by the USS Valkyrie, commanded by Captain Alexander Kurtz.
Undeterred, it appears that the Breen still intend to push for an expansion in the near future, although it is unclear what form this will take.
The concept that The Breen are multiple species united by the confederacy was considered, but ultimately, the concept had no real payoff and only proved to both demystify and complicate the Breen, which weakens their concept as a mysterious antagonist. Additionally, with the level of conflict present during the Dominion War, it is extremely unlikely that this fact wouldn’t have been established by the bodies that the Federation Alliance would inevitably have come into contact with. There are very few canon facts about their physiology, apart from the fact that they don’t have blood, and their minds are very difficult for telepaths to probe.
The Breen’s strength as an antagonist is their mysterious and aggressive nature; negotiations aren’t ever going to be very successful and the best the Federation can hope for is a stalemate, and the avoidance of open warfare. Additionally, keeping them isolated means that it is much easier to write them into antagonistic roles without the likely consequence of a quadrant-scale war breaking out.