The Breen-Dominion Alliance was an alliance formed between the Breen Confederacy and the Dominion in 2375, during the Dominion War. It was an initially successful attempt to gain an advantage over the Federation Alliance, but also contributed to the Cardassian Rebellion and, ultimately, the Dominion's defeat.
Prior to the alliance, there was conflict between the Dominion and the Breen. In 2373, the Dominion was holding a Breen prisoner at Internment Camp 371. (DS9: "In Purgatory's Shadow", "By Inferno's Light") In an incident that year, the Breen prisoner killed two Jem'Hadar guards but was killed in retaliation. (DS9: "By Inferno's Light") The next year, Vorta clone Yelgrun commented that, before he had encountered Ferengi, he had considered Breen annoying, though he now found them less so than Ferengi. (DS9: "The Magnificent Ferengi")
After the Romulan Star Empire joined the Federation Alliance in 2374 and the First Battle of Chin'toka followed, the Dominion lost the initiative in the war. (DS9: "In the Pale Moonlight", "Tears of the Prophets") Therefore, the Female Changeling, residing at Dominion Headquarters on Cardassia Prime, decided to open talks with the Breen leadership under Thot Gor. (DS9: "Strange Bedfellows") Secretly, she neither knew nor cared about what would please the Breen; the only way they had any importance to her was if they fought for the Dominion and won, as she was desperate for the Dominion to prove victorious in the war. (DS9: "The Changing Face of Evil") The Founder notified Vorta leader Weyoun that she wanted a subspace com-link installed in her quarters, enabling her to use a secure channel, separately encoded and isolated from the main communication array. (DS9: "Penumbra") Although this was actually so she could converse with the Breen, both she and Weyoun kept the details pertaining to the contents and recipient of these communications secret from the Cardassians, even from the leader of the Cardassian Union, Legate Damar. (DS9: "Penumbra", "Strange Bedfellows") Thus, when Weyoun relayed the Founder's request to Damar, the Cardassian leader was curious as to who the Female Changeling would be speaking to and why secrecy was necessary, but Weyoun deflected Damar's questions by saying these issues were none of the Cardassian's business. The equipment was delivered and the Female Changeling found it "adequate," so the negotiations began. They were conducted in secret because she felt that the Cardassians, in spite of their unity with the Dominion, should not be trusted. (DS9: "Penumbra", "Strange Bedfellows")
The Breen and the Dominion arranged rendezvous coordinates for the historic first face-to-face meeting of the two societies. Weyoun believed the Female Changeling's plan of recruiting the Breen as allies of the Dominion was "brilliant" and would change the course of the war to favor their side instead of the Federation Alliance. He, Damar, the Female Changeling, and a crew of Jem'Hadar took Weyoun 7's attack ship, a Jem'Hadar fighter, to meet with the Breen. Even when the warship was merely thirty-six hours away from the rendezvous point, the Female Changeling still harbored apprehensions that Damar would interfere in the plans for the alliance, though Weyoun assured her he would deal with the Cardassian leader when the time arrived. Despite also telling the Female Changeling that he expected Damar would have considerations regarding the alliance, Weyoun predicted to Damar himself, when he demanded to know where the ship was heading, that he would be "very pleased" with the news. (DS9: "'Til Death Do Us Part")
When the Jem'Hadar fighter and a Breen ship both arrived at the rendezvous site, a Breen delegation, led by Thot Gor, beamed aboard the Jem'Hadar vessel. (DS9: "'Til Death Do Us Part") Since the Breen had recently abducted Ezri Dax and Worf on the planet Goralis, the group of Breen brought those two prisoners with them, as gifts for the Dominion. (DS9: "Penumbra", "'Til Death Do Us Part") The newcomers to the Jem'Hadar spacecraft were met by Weyoun, Damar, and some of the Dominion warship's complement of Jem'Hadar soldiers. Weyoun gratefully accepted the pair of "gifts" and announced the birth of the alliance between the Breen and the Dominion, observing to the captives that it changed "everything." (DS9: "'Til Death Do Us Part") In fact, in his mind, the Breen lost their individuality when they joined the Dominion. After proclaiming the start of the pact, he introduced Gor to Damar, but there were communication difficulties between them, as Damar hadn't yet had a chance to adjust his universal translator to account for the Breen language. The Female Changeling shapeshifted just before she arrived, so that the Dominion's Breen guests wouldn't see that she was actually suffering from a morphogenic virus, and she then greeted the Breen.
At this point, the alliance treaty documents were still to be signed by both interstellar powers. The Female Changeling reckoned that, if the face-to-face talks between herself and Gor turned out to be as successful as their subspace discussions had been, these documents would be signed by them within a few days. She was confident that the alliance would finally bring a satisfactory conclusion to the Dominion War, as she was convinced that the addition of the Breen would result in the Federation becoming unable to oppose the Dominion and therefore being eliminated from the galaxy.
The alliance treaty included several unspecified territorial concessions of the Cardassian Union favoring the Breen, listed in a secret protocol known only to the Breen and the Dominion, but not to Damar. At first, the fact he hadn't even seen the treaty before it was discussed between Gor and the Female Changeling annoyed Damar, yet Weyoun gave personal assurances that he and Damar would speak about it later. When that time came, the particulars of the treaty, when Damar read it on a PADD, outraged him even more. The treaty hadn't been read in its entirety by Weyoun. Nonetheless, he held the opinion that the Dominion needed the aid of the Breen to win the war and that all Damar was required to do was simply to sign the document as the acknowledged leader of the Cardassian Union. Damar asked to see the protocol, but Weyoun refused him access and arranged for Damar to sign the documents as well as to submit them to Weyoun within an hour.
Meanwhile, Worf felt it was imperative for him and Ezri Dax to return to Federation space station Deep Space 9 so they could alert Starfleet about the Dominion and Breen having forged an alliance. The Dominion was granted information which the Breen had extracted from the imprisoned couple, although the information was somewhat confusing. The Breen, on the other hand, were permitted admission to the command center at Dominion Headquarters. The Female Changeling even decided they would be allowed unlimited use of an otherwise classified Dominion database. However, Damar was aggravated by the Breen being given any access to the database, and argued that the Breen didn't have to be made aware of all the Dominion's military operations in order to assist the Dominion. Another arrangement between the two parties, again decided on by the Female Changeling, was that Damar would report his military recommendations to Thot Gor, an instruction Damar profusely objected to. (DS9: "Strange Bedfellows") Much as they were willing to cooperate with the Dominion, the Breen would not disclose exactly why they wore refrigeration suits, which puzzled Weyoun because he had determined their home planet actually wasn't as cold as Dominion intelligence reports suggested it was. (DS9: "The Changing Face of Evil")
Damar's frustration over the specifics of the union led him to start a Cardassian rebellion against the Dominion and enable Worf and Ezri Dax to escape. (DS9: "Strange Bedfellows") Once they were back on DS9, their commanding officer, Captain Benjamin Sisko, wanted them to tell him about the new alliance immediately. Although Worf and Ezri explained that they themselves knew very little about the development, Dax did reveal to Sisko that she suspected Damar feared it would weaken Cardassia's status with the Dominion even more. (DS9: "The Changing Face of Evil")
The Breen entered the war on the side of the Dominion by committing a surprise attack on Earth, which devastated Federation morale. The alliance imperiled not only Earth but also the Cardassian rebellion, for whom the partnership with the Breen made the Dominion even more formidable than ever. Damar attempted to warn Thot Gor that, if the alliance failed to quickly gain final victory in the war, the Dominion would end up shifting blame from the Cardassians to the Breen. The seriousness of how the current situation affected the Federation caused Quark, who also learned of the recently established alliance, to be alarmed to find Miles O'Brien and Julian Bashir essentially "playing games," reenacting the Battle of the Alamo with scale models.
Regardless of Damar's cautioning, the Breen persisted; a Dominion fleet in which the Breen participated launched a battle to retake the Chin'toka system shortly thereafter, with the Breen playing a crucial role in the confrontation. Nevertheless, while the engagement was underway, they weren't present on the bridge of Weyoun's attack ship, from which the Female Changeling, Weyoun, and Jem'Hadar oversaw the fight. Thanks to a type of Breen energy dissipator, the dispatched Federation Alliance relief fleet was destroyed save for one Klingon Bird-of-Prey, the IKS Ki'tang. At the Female Changeling's request, the Dominion thanked the Breen for this victory.
Soon afterwards, both factions, with their leaders gathered at Dominion Headquarters, witnessed Damar officially declare that the Cardassians were separating from the Dominion and initiating a rebellion against them. Incidentally, this announcement was received while the Female Changeling was allowing the Breen to see her true infected appearance. As a result of the Cardassian declaration, the Dominion tasked the Breen with hunting Damar down. (DS9: "The Changing Face of Evil") By that point, the Breen were also defending numerous Dominion facilities. (DS9: "When It Rains...") The Dominion wasn't expecting the Federation Alliance to reciprocate with another attack so soon after the Breen entered the war, so when the Klingon Empire carried out a raid on Avenal VII, it took the Dominion by surprise, but they managed to defeat the Klingons on that occasion. In defiance of Weyoun attempting to assure them that the Cardassian uprising posed no threat to them either, the Female Changeling and Thot Pran, a Breen official, argued otherwise. (DS9: "Tacking Into the Wind") For this reason, the Breen, under orders from the Female Changeling, had the energy weapon installed on Dominion ships (such as at the Kelvas facility), though the Federation Alliance eventually managed to develop a countermeasure to the weapon. (DS9: "Tacking Into the Wind", "The Dogs of War")
When Legate Broca was announced as the new leader of the Cardassian Union, the leaders of the Dominion and the Breen, believing Damar had been killed and the Cardassian rebellion had been crushed, met Broca, who eagerly anticipated working with the Breen Confederacy. (DS9: "The Dogs of War") The Female Changeling ensured the allegiance of the Breen by making empty promises to them. For instance, shortly before the Battle of Cardassia, she vowed to give them control of both Romulus and Earth after victory, though she implied towards Weyoun that she didn't intend to keep her promises and admitted she would even guarantee the Breen all of the Alpha Quadrant if she thought it might motivate them to win the war.
After it was discovered that Damar was still alive, every Jem'Hadar and Breen soldier on Cardassia searched for him. At one point in this major undertaking, Colonel Kira Nerys disguised herself as a Breen, wearing one of their refrigeration suits, and shot down a pair of Jem'Hadar. In reality, the Breen and the Jem'Hadar stood united even on the battlefield, albeit with the Female Changeling at first expecting that the Jem'Hadar's morale would be damaged if the Breen were typically situated ahead of them. Under pressure from Thot Pran, she authorized the Breen to stand together with the Jem'Hadar as first line of defense for the Battle of Cardassia, with Thot Pran voluntarily leading them into that last battle of the Dominion War. While the encounter raged, Broca, Damar, and Weyoun were separately killed on Cardassia.
Whereas Colonel Kira subsequently tried to persuade the Female Changeling into ordering the Jem'Hadar and Breen to concede defeat, the alliance withstood Kira's appeal to the Founder, who defiantly replied that the Breen and Jem'Hadar would fight to the last man. After Odo influenced the Female Changeling to acquiesce, however, the war ended with the surrender of the Breen-Dominion Alliance and the signing of the Treaty of Bajor. (DS9: "What You Leave Behind")
The term "Breen-Dominion Alliance" comes from a statement Benjamin Sisko makes in the teaser of "The Changing Face of Evil". In the first draft of that episode's script, however, Sisko instead called it the "Dominion-Breen Alliance".
The Breen-Dominion Alliance was conceived quickly, intended to be portrayed as a result of the Dominion having lost the support of the Romulan Star Empire. "We were flying by the seat of our pants with this," recalled Executive Producer Ira Steven Behr. "We knew that bringing in the Breen would up the stakes. But we didn't have the whole thing worked out." (Star Trek: Deep Space Nine Companion (p. 685))
Star Trek author Christopher L. Bennett speculated that "the main reason" why the alliance was invented by the writing staff of Star Trek: Deep Space Nine was "to establish that the Dominion saw Cardassia as disposable – not the partners they'd promised, but just a stepping-stone toward their conquest of the quadrant, to be tossed aside as soon as they'd outlived their usefulness (if you'll pardon the mixed metaphor). It was a motivator for Damar's – and Cardassia's – journey toward rebellion." 
The DS9 writing staff decided to set up the alliance by having the Breen capture Ezri Dax and Worf. "It was a very roundabout way of getting to a larger geopolitical point," stated René Echevarria, a member of the show's writing team. (Star Trek: Deep Space Nine Companion (p. 685))