The secretive Alpha Quadrant power whose refrigeration-suited soldiers joined the Dominion War in 2375.
The Breen Confederacy is the state of the Breen, a species whose members always appear in refrigeration suits and helmets that hide their faces. Long known for raiding and secrecy, it kept a hostile distance from the Federation for much of the 24th century.
In 2375 the Breen joined the Dominion and attacked Earth, striking Starfleet Headquarters in San Francisco. Their energy-dampening weapon helped destroy an allied fleet at Chin'toka in The Changing Face of Evil, until the allies found a defence against it.