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 ambassadors, 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 cloaked 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 convoys 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 starbases 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 Founders 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. ()

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de:Föderal-Klingonischer Krieg (2372/73)
ja:連邦・クリンゴン戦争（2372年～73年）
nl:Federatie-Klingon oorlog (2372-73)
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