The First Battle of Deep Space Nine opened the Second Federation-Klingon War of 2372-2373. In early 2372, a Klingon task force led by Chancellor Gowron and General Martok launched an attack on the Federation space station of Deep Space 9, in an attempt to force Captain Benjamin Sisko to relinquish the Cardassian Detapa Council members under his protection. The Klingons had under-estimated the station's defenses, and were unable to take the station before the arrival of Starfleet reinforcements. Faced with the prospect of massive losses and an escalating war on two fronts, Gowron ordered his forces to withdraw.
After the Battle of the Omarion Nebula in 2371, the Cardassian Union was in a state of turmoil. For centuries, the Obsidian Order had kept an iron grip on all levels of Cardassian society, keeping the status quo firmly in place. With the Obsidian Order decimated by the Jem'Hadar, the Cardassian Underground was finally able to make headway against the oppressive rule of the Central Command. Faced with growing unrest and civil uprisings, the Central Command sealed the borders of the Cardassian Union to focus their attention on the rebellion. Despite their efforts, early in 2372 a popular revolt overthrew the Central Command and restored the power of the civilian Detapa Council.
Meanwhile, the Klingon Empire had been infiltrated by a Founder, masquerading as General Martok. This Founder, taking advantage of the paranoia spread by the changelings, influenced the Klingon High Council into concluding that the Cardassian uprising could have succeeded only with Dominion assistance -- and, ironically, that the Detapa Council had been replaced by Founders. Martok advocated that the Klingons mount an invasion of the Cardassian Union to neutralize the "Dominion threat". His proposal came at a time when many Klingons believed that years of peace had led them astray, and thus the invasion plans were eagerly embraced by Klingons hoping for a return to the gloried old ways of war and conquest.
When preparations were complete, a Klingon task force of over fifty warships, led by the IKS Negh'Var, arrived at Deep Space 9 claiming to be supporting the Federation in their stand against the Dominion. Once their true purpose was discovered, Captain Benjamin Sisko urged Martok to reconsider their plans. Heedless of Starfleet's refusal to support them, the Klingon fleet proceeded into Cardassian space, commencing Klingon-Cardassian War. Bound by their alliance, the Federation could do nothing to interfere, although Sisko "unofficially" sent an advance warning to the Cardassians through Elim Garak. A few days later, the Federation Council condemned the Klingon invasion. Outraged that his ally had refused to support him in "defending the Alpha Quadrant" from the Dominion, Gowron withdrew from the Khitomer Accords and broke off all diplomatic contact.
Flight from Cardassia
Although the Cardassian fleet mobilized in time thanks to Garak's warning, the speed and magnitude of the Klingon advance broke through their lines a short time after the invasion began. Now the Klingon forces were only 52 hours from Cardassia Prime itself, and if they were permitted to conquer the planet and execute the Detapa Council, the war would effectively be over. To prevent this from happening, Sisko contacted Gul Dukat, the new Chief Military Advisor to the Detapa Council, and advised him to evacuate the Council to Deep Space 9. Dukat agreed, and left Cardassia with the Council members aboard the cruiser Prakesh.
At the same time, Sisko took the USS Defiant into Cardassian space to rendezvous with Dukat, using its cloaking device to hide from the Klingon forces (in violation of the terms of its loan from the Romulans). When they arrived they found that the Prakesh had been intercepted by three birds-of-prey, and was under heavy attack. The Klingons ignored Sisko's order to stand down, and opened fire on the Defiant. With those shots, two decades of peaceful co-existence came to an abrupt end.
The Defiant engaged the birds-of-prey, buying time for the stricken Prakesh while a Vor'cha-class attack cruiser enters the battle. Using a modified tractor beam to deflect some of the cruiser's fire, the Defiant lowered its shields long enough transport the Detapa Council members board. The Prakesh exploded shortly afterward as the Defiant headed back for Deep Space 9 at maximum warp, the two remaining Klingon warships in close pursuit. With this development, the main Klingon force turned away from Cardassian Prime and headed to the Bajor system. Aboard the Defiant, Doctor Julian Bashir administered blood screenings of the Council members and verified that they have in fact not been replaced by Founders.
When the Defiant reached Deep Space 9, they found several dozen Klingon warships in attack formation, waiting for them beyond the station's shield perimeter. Gowron and Martok, aboard the Negh'Var, sent an ultimatum to Sisko: surrender the Council members, or have them taken by force. Sisko refused, informing them of the results of the blood screenings. Not surprisingly, the fact that the Council members were not changelings meant little to Gowron, who asserted that the Alpha and Beta Quadrants would be safer with the Klingons in control of Cardassia. Sisko gave a last warning: that in the past year, the station has had ample time to prepare for a massive attack by the Dominion or any other hostile force. These preparations included the installation of new torpedo launchers and phaser emitters, as well as a stockpile of five thousand photon torpedoes. Gowron scoffed at this threat and ordered his forces to advance, announcing "CHEGH-chew jaj-VAM jaj-KAK" -- "Today is a good day to die."
The first wave of Klingon ships open fire on the station, moving to surround it on all sides. The station's weapons returned fire, destroying eight Klingon warships on the opening volley and heavily damaging several more. Undeterred, the Klingon forces pressed on their attack. Realizing the futility of additional negotiations with the Klingons, Sisko ordered the station to fire at will. Surprised but undaunted by the station's upgraded defenses, Martok ordered the Negh'Var and several support cruisers to concentrate their fire on the station's shield generators. The Negh'Var's heavy disruptor cannons scored several direct hits, disabling two of the generators and opening a gap in the station's shields.
Seizing this opportunity, Klingon boarding parties begin materializing in the Operations Center, Promenade, Habitat Ring, and Lower Pylon 3. Small-scale firefights and hand-to-hand combat erupted throughout the station, although civilian casualties were minimal as Sisko had wisely evacuated non-essential personnel to the emergency shelters prior to the battle. The Klingon forces additionally found themselves hindered by force-fields erected between all major access points of the station. With their advanced preparations, the Starfleet and Bajoran officers were able to contain the initial Klingon incursion. Sisko and the other command staff regain control of Ops, allowing Chief Miles O'Brien to restore Deep Space 9's shields and prevent additional Klingon troops from reinforcing their positions on the station.
As the Klingon attack faltered, a Starfleet relief force of six starships, led by Admiral Hastur aboard the USS Venture, arrived to support Deep Space 9. Sensing that he was losing the advantage, Gowron signalled his own reserves to join the battle. However, Sisko warns him that to fight a protracted, all-out war with both the Cardassians and the Federation would only serve to weaken them all, paving the way for the Dominion conquest. Martok insisted that the Klingons could still prevail, but Gowron recognizes the truth of Sisko's argument and ordered the Klingon fleet to stand down: the battle was over.
The battle at Deep Space 9 ended indecisively, as even though the Klingons had failed to capture Deep Space 9 or the Detapa Council, their offensive against the Cardassians continued. After the battle the Detapa Council was safely returned to Cardassia Prime, though their rule would be short. Less than two years later, the devastating war with Klingons would lead Dukat into delivering the Cardassians into the hands of an even more oppressive power than the Central Command.
For the Federation, an all-out war with the Klingons had been averted, but the alliance that had unified the two powers in friendship for decades was lost. The ensuing Federation-Klingon conflict would end with a cease-fire a year later, after Martok was exposed as a changeling, but not before a string of largely pointless skirmishes had been fought along their border. Beginning with the Battle of Deep Space 9, the Founders had nearly succeeded in their goal of irreparably destroying the Klingon-Federation alliance, the only power which could have stood against a Dominion invasion. Fortunately for the Alpha and Beta Quadrants, the two powers were able to reconcile in time, but until not after the loss of a great many lives.