The Second Battle of Chin'toka marked the entry of the Breen Confederacy into the Dominion War in late 2375, in an attack against the Chin'toka system, a territory of the Cardassian Union held by the forces of the Federation, the Klingon Empire, and the Romulan Star Empire. The battle was a major tactical defeat for the allies — and perhaps one of the most lopsided victories of the entire war, thanks to the use of the devastating and previously-unknown Breen energy dampening weapon. (DS9 "The Changing Face of Evil")