Second Federation-Klingon War
The war took place as a consequence of Dominion attempts to undermine the long-standing peace between the two powers. The Founders realized that a Federation-Klingon alliance was certain in the face of a Dominion invasion, and thus they sought to seed discontent throughout the Beta Quadrant. When a coup on Cardassia Prime shifted power from the Central Command back to the Detapa Council, the Klingon Chancellor Gowron was certain it was a Dominion plot. He ordered an invasion of Cardassia in 2372.
The Federation, however, refused to take part and denounced the invasion. In return, Gowron withdrew the Empire from the Khitomer Accords, ending the peace. By late 2372, the Klingons continued hostilites toward Cardassia Prime and even began attacking Federation, Romulan, and Bajoran interests. Gowron then demanded the Federation withdraw from the Archanis Sector along the border within ten days. When the Federation refused, the Klingons attacked in full-force and poured over the border. Fighting broke out all over, even as far as the Bajoran Sector. Starfleet was wholly unprepared, as they had steadily withdrew ships and stations along the border after the Accords.
Within a few months, however, the Federation was able to stop the Klingon advance and in some sectors, push them back. Intelligence would later convince the Federation that Gowron had been replaced by a Founder. A covert mission led by Capt. Benjamin Sisko infiltrated the Klingon military headquarters with orders to expose the Gowron Changeling. In fact it wasn't Gowron but rather his trusted aid Martok that was the Founder. In exchange for exposing the spy, Sisko and his team was allowed to leave Klingon space and a cease-fire followed.
Unfortunately, the cease-fire did not last long. A few weeks later, the Klingons attacked Ajilon Prime in response to the supposed Federation victory at Ganalda IV. Starfleet took advantage of the reinstatement of hostilities and launched an attack to retake the Archanis Sector. Despite heavy losses on both sides, neither were able to gain an clear advantage. The cease-fire was again put back in place.
The war officially ended six months later, when Chancellor Gowron re-signed the Khitomer Accords in the face of a Dominion invasion.
(References: Deep Space Nine: "The Way of the Warrior", "Broken Link", "Apocalypse Rising", "Nor the Battle to the Strong", "By Inferno's Light")