From Hidden Frontier
During the Dominion War, the Breen Confederacy allied with the Dominion against the Federation Alliance. In 2375, the Breen led an attack on Earth, which damaged Starfleet Headquarters and the city of San Francisco, but the USS Angeles led planet's defense.
The Breen Confederacy chose to isolate itself from the rest of the Quadrant. A Federation diplomatic team was sent to the Confederacy. (ANG 1.01 - "Return to Duty") The Breen attempted to terraform Ba'ku by placing sub-methane cooling units in the planet's mantle, but the plan failed. After rogue agents attempted to force a colony on the planet, the Breen were allowed to send three scientists to the planet's surface. (ANG 1.03 "Dreamers" and HF 2.06 - "To The Stars")
The Breen sent a fleet of warships to the planet, Ayre, which was discovered to have Iconian technology on the surface of the planet. This first fleet was destroyed by the Iconian vessel, Defender, in orbit of the planet. The Breen sent a second fleet which was defeated at the Battle of Ayre by the Defender and the USS Angeles to prevent the Breen from acquiring the technology. (ANG 1.05 "The Last of the Iconians")