The Breen Confederacy was the isolationist government of the Breen, located in the Alpha Quadrant. Shrouded in mystery, the Breen Confederacy were one of the most underestimated powers that quadrant of space. In late 2375, the Breen Confederacy entered the Dominion War by signing an alliance with the Dominion.
In addition to Breen itself, Portas V was part of the Breen Confederacy. (DS9: "For the Uniform") By the 2370s, the Confederacy maintained diplomatic or trade relations with the Ferengi and the Cardassians. (DS9: "Ferengi Love Songs", "Indiscretion")
According to the terms of the alliance, the Breen Confederacy received several planets from the Cardassian Union as compensation for joining the war. The Breen would also be given control of the Romulan Star Empire in the post-war galaxy. The Founders also promised the Breen control of Earth, though Weyoun was also given this assurance. The alliance was negotiated between the Female Changeling and Thot Gor, a prominent official in the Confederacy. (DS9: "Strange Bedfellows")
The Confederacy proved very influential in its first weeks of fighting, thanks to their previously unknown energy-dampening weapon, in addition to their conventional forces. The weapon proved decisive in the Second Battle of Chin'toka, allowing the Dominion to retake the Chin'toka system after inflicting huge losses on the Allies. This success followed up on a surprise attack on Earth; both events significantly damaged allied morale. (DS9: "The Changing Face of Evil")
However, the Allies were able to capture an example of the Breen weapon, allowing them to develop countermeasures. The Breen continued to fight on the side of the Dominion until the end of the war, demonstrating a willingness to die for the cause comparable to that of the Jem'Hadar. In some respects this dedication actually worked to the benefit of the Allies – the Dominion's increasing reliance on the Breen forces made the Cardassians feel increasingly isolated and abandoned, and was a contributory factor in the Cardassians' eventual split from the Dominion. (DS9: "Tacking Into the Wind")