In 2375 Dominion and Breen forces struck the Allied fleet at Chin'toka, where a Breen energy-dissipating weapon disabled the Allied ships and the Defiant was destroyed.
After the Breen joined the Dominion in 2375, the Allies tried to hold the Chin'toka system, which they had taken the year before. A combined Dominion, Cardassian and Breen fleet attacked them there. The Breen used a new weapon that drained the power from Federation, Klingon and Romulan ships, leaving them helpless.
The Allied fleet suffered huge losses, and the USS Defiant was destroyed, though its crew escaped. One Klingon ship was unaffected by the weapon, which gave the Allies their first clue to how to counter it.
The battle made it seem that the Dominion might win the war after all. The Allies were forced onto the defensive, and the Klingons alone could fight on while a defense was found. The defeat also pushed the Federation to rely more on Damar's Cardassian rebellion against the Dominion.