The alliance made in 2375 between the Breen Confederacy and the Dominion, which brought a powerful new fleet and an energy-draining weapon into the war against the Federation and its allies.
The Breen-Dominion alliance is revealed in the final run of Deep Space Nine episodes, starting with Penumbra and 'Til Death Do Us Part. Worf and Ezri Dax are captured by the Breen and handed to the Dominion as a gift, and in Strange Bedfellows the Female Changeling signs a treaty with the Breen commander Thot Gor.
The Breen are promised territory, some of it Cardassian, and are given authority over Cardassian forces. Weyoun did not consult Legate Damar about any of this, and the insult helps to push Damar into rebellion.
The new allies raid Earth and damage Starfleet Headquarters, then win the Second Battle of Chin'toka with a weapon that drains a ship's power, where the Defiant is lost. Only one Klingon vessel proves immune, and Klingon ships hold the front until a defence is devised. The Breen fight on until the surrender at Cardassia Prime.