In early-2372, the Cardassian Dissident Movement succeeded in its goal remove the Cardassian Central Command from power and install the Detapa Council as the ruling body of the Union. The Klingon Empire came to believe that this was due to Changeling interference and prepared their forces to secure the Cardassian Union.
Launching a major invasion under the command of General Martok. Despite prior warning from Garak the Union was still not prepared for the massive invasion and the outer planets fell to the Klingons. They rallied and slowed down the Klingon advance but the Klingons eventually broke through their lines and made their way towards Cardassia Prime.
Captain Benjamin Sisko arranged with Legate Dukat to evacuate the Depata Council and rendezvous with the USS Defiant. Despite been attacked by Klingon forces Sisko was able to rescue the Council and evacuate them to Deep Space 9.
In response Gowron ordered his forces to attack the station to capture the Council. The arrival of Federation reinforcements made Gowron reconsider and pull his forces back than risk a war on two fronts. Gowron then declared the invasion a success and the Klingons fortified their captured territories.
The war entered a period of prolonged stalemate. Klingon raids continued in the Cardassian Union keeping the Cardassian Guard on the back foot and debilitating the Union's economy. The Klingons also started to covertly support the Maquis by providing them with equipment, including 30 class 4 cloaking devices.
While the Federation did not enter the conflict as a combatant the cancelling of the Khitomer Accords by Gowron in response to the Federation Council denouncing the invasion saw increased tensions between the two powers.
The Federation provided economic and humanitarian assistance to the Cardassian Union during the duration of the conflict. Starfleet provided escorts for several non-military convoys, which saw Federation and Klingon vessels exchange fire.
Dominion intervention and the end of the war
At some point a demoted Gul Dukat began to secretly negotiate with the Dominion. In mid-2373, a Dominion fleet entered the Alpha Quadrant and supported a coup d'état in the Cardassian Union that saw Dukat installed as its new ruler. Dukat immediately declared the Cardassian Union would join the Dominion as a member.
With their new partnership the Dominion quickly went about securing their new territory. The Jem'Hadar reinforcements brought from the Gamma Quadrant quickly made their mark and drove the Klingons out of the territory they had captured, effectively ending the war.
The Dominion's arrival in force in the Alpha Quadrant and their quick defeat of the Klingons saw the Federation and Klingon Empire reinstate the Khitomer Accords. Despite this show of unity the Dominion was able to make significant strides into the region; signing several non-aggression pacts, including with the Romulan Star Empire.
The Dominion continued to build up their military presence in Cardassia with weekly convoys. This eventually resulted in a counteraction by Starfleet with the mining of the wormhole and would lead to war.