Federation - Klingon War ( 2372 - 73 ) The Federation - Klingon War was a brief but intense conflict fought primarily between the United Federation of Planets and the Klingon Empire , from late 2372 to mid - 2373 . Intermittent hostilities , and later open war , between the two powers followed from the breakdown of the Federation - Klingon Alliance in the wake of the Klingon - Cardassian War , which were engineered by Changeling agents to weaken the Alpha Quadrant . A fragile cease fire was signed after the involvement of the Dominion was exposed , but the resumption of the alliance did not occur until the Dominion made a full - scale incursion through the Bajoran wormhole . Background . In the early 2370s , the Dominion sent several Changeling agents to weaken the Alpha Quadrant powers in preparation for an invasion . drew the Romulan Tal Shiar and the Cardassian Obsidian Order into a trap in the Omarion Nebula in 2371 . With these organizations decimated , the last bastion of resistance to the Dominion was the Federation - Klingon Alliance . ( ) Freed from the watchful eye of the Obsidian Order , the Cardassian dissident movement succeeded in toppling the Central Command in early 2372 and restoring power to the civilian Detapa Council . , having replaced the prominent Klingon General Martok , convinced Chancellor Gowron that the " coup " had been a Dominion plot and that the Klingon Empire must conquer the Cardassian Union for the sake of the quadrant ' s security . ( ) Breaking the alliance . Starfleet refused to support the Klingons ' aggressive actions , which included the searching of all ships leaving Bajoran space for Changelings . One such search of the Federation freighter nearly led to an exchange of fire between the and the . The Federation Council , failing to dissuade the Klingons , condemned their invasion of Cardassia . Gowron retaliated by withdrawing from the Khitomer Accords , recalling his ambassador s , and expelling all Federation citizens from Klingon territory , thus ending the alliance between the two governments . Captain Benjamin Sisko rescued the Detapa Council from the Klingons , which led to a massive Klingon assault on Deep Space 9 . The Klingons were repelled by the station ' s upgraded defenses and the arrival of Starfleet reinforcements . ( ) However , Gowron continued to see the Federation as weak and war inevitable . ( ) In the following months , the Klingons continued to prosecute their war with the Cardassians while escalating hostilities along the rest of their border , probing for exploitable weaknesses . ( ) They also attempted to secretly place a cloak ed minefield that would isolate the Bajoran system ; the minefield was destroyed by the USS " Defiant " with the assistance of Kurn . ( ) The Federation Council kept pressing the Klingons to return the Cardassian colonies they had seized , which was angrily rejected . ( ) Klingon and Federation vessels clashed again when Starfleet agreed to escort seven Cardassian humanitarian convoy s bound for the Pentath system . In an attempt to discredit Starfleet and force it to cease its support of the Cardassians , the Klingons staged a " massacre " wherein a Klingon civilian transport decloaked in front of the " Defiant " amidst a battle , leading to its destruction by Lieutenant Commander Worf ' s orders . The deception was exposed by Odo , who uncovered evidence that the transport was in fact empty . ( ) Open war . Near the end of 2372 , the Martok Changeling influenced Gowron into reasserting the Klingon Empire ' s claim to the Archanis sector , which it had relinquished to the Federation a century prior . Gowron sent an ultimatum demanding that the Federation leave the sector , abandoning all starbase s and military installations . The Federation attempted to stall and negotiate , though Federation worlds along the Klingon border began calling for a preemptive strike against the Empire . Finally , Gowron dispatched a task force to conquer Archanis , giving Starfleet ten days to withdraw or be fired upon . In response , Starfleet and the Bajoran Militia prepared for war . Meanwhile , the Founder s deceived Odo into believing that Gowron had been replaced by a Changeling , hoping that Starfleet would assassinate him and leave the false Martok in control . In early 2373 , the Klingons invaded Federation space along a broad front from Archanis to Bajor , committing all available resources . Federation defenses proved inadequate and Starfleet was only able to slow their advance ; the and the were among the starships that took heavy casualties . Gowron relocated Klingon military headquarters to Ty ' Gokor , a heavily fortified planetoid . Based on Odo ' s information , Starfleet sent a team , led by Sisko , to infiltrate Ty ' Gokor with Dukat ' s captured Bird - of - Prey and expose Gowron as a Changeling . The team ultimately discovered the Changeling to be Martok , who was killed by the Klingons . Gowron was reluctant to end the war unless the Empire were allowed to annex Archanis IV and the other worlds it had captured . Nevertheless , Sisko prevailed on him to push the Klingon High Council into accepting a cease fire . ( ) The Federation and the Klingon Empire subsequently engaged in peace talks . ( ) Many Klingons continued to clamor for war despite having proof of Dominion machinations , confident the Empire would emerge victorious . ( ) The cease fire was briefly violated several weeks later when Klingon troops attacked the Federation colony on Ajilon Prime , possibly as revenge for their defeat at Ganalda IV . Many colonists were killed and the planet nearly fell before the cease fire was reinstated ; the , sent to relieve the colony , was also destroyed by the Klingons near the Lembatta Cluster . During the resumption of fighting , Starfleet began assembling a fleet to retake the Archanis sector , spearheaded by the and the . ( ) Restored alliance . In mid - 2373 , an unexpected game - changing event forced the Federation and the Klingons to resolve their conflict . Gul Dukat announced the incorporation of the Cardassian Union into the Dominion , with himself as the Cardassian leader . A Jem ' Hadar fleet crossed the Bajoran wormhole and joined the Cardassians ' fight against the Klingons . In three days , the Klingons suffered severe losses , leaving Gowron with no choice but to order a general retreat from Cardassian territory . Gowron and his battered fleet arrived at Deep Space 9 , where Sisko convinced him to revive the Khitomer Accords and make a united stand with Starfleet at the station . With the alliance restored , the Klingons established a permanent military presence on Deep Space 9 commanded by the real General Martok , who had recently escaped from Dominion Internment Camp 371 . ( ) Aftermath . Though the Founders ' disruption of the Federation - Klingon alliance proved temporary , the war they incited between the two powers was costly to both . By the time the Dominion entered the Alpha Quadrant in full force , Starfleet ' s strength had been depleted to the point that defeating the Dominion was assessed to be impossible . ( ) The two allies would later be hard - pressed to resist the Dominion advance in the opening months of the Dominion War . ( )