The Breen War was a six-year war between the Galaxy Alliance (and later the Rihannsu Stelam Empire) and the Breen. Over the course of the conflict, Starfleet suffers tens of thousands of casualties, the loss of over one hundred vessels, and the planet Beerax IX is deemed uninhabitable.
In early 2380, Starfleet forces discovered the USS Venture, a Shutsugun-class Heavy Cruiser, adrift with all one thousand crew members slaughtered aboard but with little evidence to find. Over the course of the next few months, a series of altercations occur between Starfleet vessels and unidentified ships.
In response, newly appointed Chief of Starfleet Operations, Vice Admiral Ruao Sarjanna, increased her efforts to reorganize the fleet and establishes the USS Akagi Battle Group, a fleet that included not only Ruao's flagship the USS Akagi, but also War Hero Kyle Richards' USS Normandy, a collection of the brightest military commanders throughout the Alliance, and the Alliance Flagship, the USS Odyssey.
In May 2380, it is discovered that the Breen are the culprits for the incursions and the loss of the Venture when a Breen fleet launches a devastating attack on the Beerax Sector. Though Starfleet fends off the attack, they pay for the encounter with heavy losses: 50,000 dead and 59 starships lost, including the Alliance Flagship the USS Odyssey.
The Galaxy Alliance High Council officially declares war on the Breen in June, and ramps up military operations throughout the Alliance. The Akagi Battlegroup begins active patrols along the Alliance Border from Bajor, through Beerax and Stebben, to Ferenginar.
The next major altercation occurs in October, when hundreds of Breen vessels engage the fleet at Beerax. Starfleet loses tens of thousands of servicemembers and several capital vessels but destroys every single Breen vessel in the conflict.
In response to the horrific battles, the Galaxy Alliance calls on the Rihannsu Stelam Empire to abide by the mutual defense clause of the Alliance/Rihannsu Tready and provide military assistance in the conflict. The Galae dedicates the 18th Star Wing, the Rihannsu's fleet directly dedicated to the defense of ch'Rihan and composed of 25 d'Deridex-class Warbirds and 30 d'Latta-class Light cruisers.
In December, the Breen again engage the fleet at Beerax. One vessel breaks through the fleet and directly assaults the Akagi, destroying the flagship and causing over three thousand deaths. Vice Admiral Ruao Sarjanna and Brigadier General Shra'Hawk survive the destruction but are injured. The Alliance picks up production of the new Sovereign-class Battlecruiser, sending the USS Sovereign, USS Midway and USS Sutherland to the Beerax Sector to augment the joint Starfleet/Galae fleet.
In February 2381, Breen forces make a surprise assault of Bajor, Beerax and Brethrax in a well-organized, well-coordinated strike. About one million Bajorans are killed with another two million wounded. The Capitol City of Jalanda suffers major damage. The USS Bonhomme Richard, the USS Pulsar and at least a half-dozen civilian and military auxiliary vessels from both Starfleet and the Galae are lost. The border mining colony of Brethrax V is captured by the Breen while a Breen bio-virus is released on Beerax IX, eradicating all life on the planet. Beerax IX is deemed uninhabitable.
For the next five years, Starfleet does not come in contact with the Breen. No enemy vessels enter the Beerax Sector, nor do Starfleet vessels enter the Brethrax system. During this time, Admiral Ruao retires from active service and recommends Kyle Richards to take her place. The High Council instead appoints Rear Admiral Haile Ritter to the position.
In January 2386, the Riker Task Force, led by the Kyuushuu-II Battleship USS William T. Riker and Commodore Sheila Palmer infiltrate the Brethrax Colony and liberate it from the Breen. Fifteen starships are lost, including the USS Midway and the USS Iwo Jima with 7,500 Starfleet servicemembers dead.
In June, Starfleet captures a Breen Heavy Cruiser. The vessel is found adrift and deserted. Upon being taken over, the computer broadcasts a message: "First phase of experiment is now complete. We will withdraw for now. Our thanks." The Riker Taskforce, including USS Tarawa, USS William T. Riker, USS Enterprise and USS Excalibur are involved in the capture.
The Alliance High Council begins debating ending the war in July 2386, sparking an intense debate between Vice Admiral Haile Ritter and the Council. This debate leads directly into the Freedom War.