The Dominion-Breen Alliance was formed in 2375 between the Dominion and the Breen Confederacy during the Dominion War. It was an initially successful attempt to gain an advantage over the Federation Alliance, but also contributed to the Cardassian Rebellion and ultimately the Dominion's defeat.
After the Romulan Star Empire had joined the Federation Alliance in 2374 and the subsequent First Battle of Chin'toka, the Dominion had lost the initiative in the war. The Female Changeling therefore opened talks with the Breen leadership under Thot Gor, keeping them secret even from the Cardassian Union leader Damar. (DS9: "Penumbra")
The alliance was finalized when Damar, Weyoun and the Female Changeling left Cardassia Prime to meet with Gor aboard a Breen warship. The alliance treaty included several unspecified territorial concessions of the Cardassian Union favoring the Breen, listed in a secret protocol known only to the Dominion and Breen leaders, but not Damar. (DS9: "Strange Bedfellows")
While news of this alliance was brought to Starfleet, the Breen entered the war and launched a surprise attack on Earth which severely damaged Federation morale. Shortly afterwards, the Breen launched an attack to retake the Chin'toka system. Thanks to the Breen energy dampening weapon the dispatched Federation Alliance relief fleet was destroyed save for one Klingon Bird-of-Prey. (DS9: "The Changing Face of Evil")
Meanwhile, the disregard shown towards the sacrifices of the Cardassian people in the war, and the favoritism shown towards their new Breen allies by the Dominion led Damar into launching the Cardassian Rebellion, giving the Federation Alliance precious time to resist the Dominion-Breen alliance. The Federation Alliance had adopted a defensive posture for now, though the Klingons launched some offensives into Dominion territory like the attack on Avenal VII. The Dominion also restrained from launching more offensives, waiting for the energy weapon to be installed on Jem'Hadar ships. (DS9: "Tacking Into the Wind")
Shortly before the Battle of Cardassia, the Female Changeling promised the Breen control both of Romulus and Earth after victory, though she implied towards Weyoun that she didn't intend to keep her promises. To honor the alliance, she also allowed the Breen to stand together with the Jem'Hadar as first line of defense for the final battle of the war.