Klingon-Cardassian War
The Klingon-Cardassian War was a major interstellar conflict resulting from the Klingon Empire invading the Cardassian Union, lasting from early 2372 to mid-2373. The war devastated the Cardassian state and greatly contributed to its eventual absorption into the Dominion, thus achieving what the Klingon invasion had claimed to prevent. The manipulation of the Klingons into launching an unprovoked attack on the Cardassians would prove to be one of the greatest consequences of Changeling infiltration in the Alpha Quadrant.

Revolution and paranoia 
In 2372, reports emerged through the sealed borders of the Cardassian Union that the Cardassian Central Command had been overthrown by a popular uprising, restoring power to the civilian Detapa Council. The growing strength of the Cardassian dissident movement, coupled with the loss of security due to the collapse of the Obsidian Order following the disastrous Battle of the Omarion Nebula, had at last been able to remove one of the most oppressive regimes in the Alpha Quadrant.

On Qo'noS, the news was received in a different light. Chancellor Gowron and the Klingon High Council became convinced that the revolt on Cardassia Prime was instigated by the Dominion and that the Detapa Council had been replaced by Founders. One of the most influential officers in the Klingon Defense Forces, General , called for a massive invasion of Cardassian space to end the "Changeling threat". Many Klingons, who had grown restless after decades of peace, rallied around him. They were unaware that Martok himself had been replaced by a Changeling, whose mission to weaken the Dominion's enemies was proceeding perfectly. Emperor  condemned the invasion, to no effect. ()

The invasion involved a third of the Klingon Empire's standing forces, with over a hundred ships in the first wave alone. The plan was to mount a lightning thrust into the heart of Cardassian territory and conquer the Cardassian homeworld. After a victory, traditional Klingon practice would call for the execution of all government officials, and the installation of an Imperial Overseer to put down any further resistance. The advance task force of over fifty warships, led by Martok aboard the , was dispatched to Federation starbase Deep Space 9 under the guise of defending the station against a Dominion incursion.

Preemptive invasion 
Captain Benjamin Sisko, commander of Deep Space 9, was suspicious of the Klingons' motives as there had been no significant Dominion activity near the Bajoran wormhole for months. Thus, he ordered Lieutenant Commander Worf to uncover the Klingons' true intentions. Once appraised of the planned invasion, Sisko urged Martok to call off the attack and informed him that the Federation would not support an unprovoked attack. Martok paid his warning no heed, and shortly after commenced the invasion. The Federation Council quickly condemned the invasion, and in response an enraged Gowron withdrew from the Khitomer Accords, ending the alliance between the Klingon Empire and the Federation.

With Starfleet ordered not to interfere until the Federation Council could re-establish a dialog with Gowron, the Klingons were free to cross into Cardassian space. However, Elim Garak, aboard Deep Space 9, was able to send warning to Legate Dukat, chief military advisor to the Detapa Council. Although the outlying Cardassian colonies were overwhelmed almost immediately, the Cardassian fleet did mobilize in time to offer greater resistance as the Klingons pressed closer to Cardassia Prime.

Nevertheless, the Klingons still broke through the Cardassian lines after a pitched battle, leaving them only 52 hours from Cardassia Prime. Realizing the planet could not hold, Sisko offered to shelter the Detapa Council aboard Deep Space 9. Dukat evacuated the council members aboard the cruiser Prakesh, though they were intercepted by several Klingon warships. With a timely rescue from the , the Council members reached Deep Space 9. With this development, the Klingon fleet turned away from Cardassia and attempted to take the council members from Deep Space 9 by force. Fortunately, the weapons of the station had been heavily augmented in anticipation of a Dominion invasion, and it was able to withstand the siege long enough for Starfleet reinforcements to arrive.

Stalemate 
Following their defeat at Deep Space 9, the momentum of the Klingon invasion was blunted and Cardassia Prime was out of danger. Gowron, fearing assassination from his failure to conquer the Cardassians, simply declared victory and halted the advance of his forces. Meanwhile, the Klingons, refusing to relinquish the colonies they had conquered, began fortifying their positions and deploying orbital defense systems in preparation for a longer war of attrition. ()

In the following months, cloaked Klingon warships continued to rampage throughout Cardassian space alone or in small groups, destroying targets of opportunity and plundering Cardassian shipping. The outmatched Cardassians responded with unconventional tactics such as sensor ghosts and holo-projections, which earned the respect of some Klingons. ()

The Klingon attacks devastated the Cardassian infrastructure, resulting in a humanitarian crisis. Diseases once under control swept through Cardassian colonies, while the government struggled to even meet the basic needs of its people. With their fleet occupied, the Cardassians were forced to ask the Federation for assistance, such as in providing escort vessels for medical relief convoys responding to an epidemic of Rudellian plague in the Pentath system. The Federation also provided twelve CFI replicators to help rebuild Cardassian industries, though they were hijacked by Deep Space 9's chief of Starfleet security, Lieutenant Commander Michael Eddington, who was secretly a member of the Maquis. The war had virtually granted the Maquis free reign in the Demilitarized Zone, as the Cardassians no longer had the resources to combat them. ()

Despite their losses, the Detapa Council remained unwilling to lose what was left of its fleet and remained on the defensive, hoping for a diplomatic solution. After the destruction of the outpost on Korma, which also killed fifteen Bajoran delegates attending a conference, Gul Dukat and Major Kira Nerys managed to capture a Klingon Bird-of-Prey. Unable to convince the Council to go on the offensive with the trove of military intelligence provided by the Bird-of-Prey, Dukat decided to embark on a personal war with the Klingons. ()

Cardassia joins the Dominion 

As the war dragged on ineffectually into a second year, Gul Dukat entered secret negotiations for the Cardassians to join the Dominion, placing himself at the head of the new Cardassian government. In mid-2373, a massive Jem'Hadar fleet of over fifty ships crossed the wormhole and entered Cardassian space. In only three days, they inflicted massive casualties on the Klingon forces. Gowron was forced to issue a general retreat, intending to fortify the Klingon Empire and prepare for a fight to the death. Instead, Sisko prevailed on him to revive the Khitomer Accords aboard Deep Space 9 and stand together with the Federation against the Dominion. After over a year of conflict, Klingon and Cardassian territories had returned to their pre-war state, only with the Klingon military substantially weakened, making the Dominion position that much stronger for the coming Dominion War. ()

External link 

de:Klingonisch-Cardassianischer Krieg
ja:クリンゴン・カーデシア戦争
nl:Klingon-Cardassian oorlog
pl:Wojna Klingońsko - Kardasjańska
Conflicts