Outbreak and early engagements
The outbreak of the war was marked by the Battle of the Binary Stars on stardate 1207.3. This battle was provoked by T'Kuvma, a Klingon from the House of T'Kuvma, to rally the Empire to a common cause as he had portrayed the Federation as an existential threat, whose space had encroached unacceptably close upon that of the Klingons. (DIS: "Battle at the Binary Stars")
Captain Gabriel Lorca of the USS Discovery claimed that, sometime after the war began, he was given the freedom to do whatever it took to win the war, even if it meant breaking the rules of Starfleet. As part of this effort, Starfleet co-opted research by Paul Stamets and Straal into an experimental spore drive. Six months into the war, the Discovery managed to practically implement the drive and went on to achieve a string of victories. This was made possible by capturing an alien called a tardigrade. (DIS: "Context Is for Kings", "Choose Your Pain")
In November of 2256, a Klingon force managed to slip past the blockade defending Corvan II, a major resource hub whose mines produced forty percent of the dilithium supplies in the Federation. Without the planetoid, a sizable part of Starfleet would be grounded. By the time the colony managed to get out a distress call, the planetary defense shields would only hold another six hours under the continued Klingon bombardment.
After their first success with the spore drive, Discovery quickly became the most important weapon in the Federation arsenal, causing the tide to turn and the Federation to start winning. In the three weeks following the relief of Corvan II, Discovery further used its drive to break the Klingon supply line at Benzar and rout an attack through the Ophiucus system. Starfleet started building spore drive units in a secret facility on Earth, while directing every ship, colony, and starbase to search for more tardigrades. (DIS: "Choose Your Pain", "Magic to Make the Sanest Man Go Mad")
Around this time, General Kol established a new ruling council. Starfleet believed that he had ousted the House of D'Ghor and the House of Mo'Kai, who secretly approached Vulcan to discuss a potential alliance. However, this was, in fact, a ruse by Kol to capture a high-ranking Vulcan. The plan worked better than expected when, due to unforeseen circumstances, Admiral Katrina Cornwell arrived in the place of Sarek. After Cornwell's capture, Kol welcomed Dennas' and Ujilli's Houses into his Empire and promised them cloaking technology. (DIS: "Lethe")
As more Houses joined General Kol, the number of ships utilizing cloaking technology increased, and Klingon activity became more aggressive. A group of Klingon vessels utilized the new advantage to destroy a group of Starfleet vessels on patrol simultaneously, including the USS Gagarin. Most disturbingly was that this advantage neutralized the tactics of the spore-equipped Discovery, which arrived to assist the Gagarin but could not save the stricken ship. (DIS: "Si Vis Pacem, Para Bellum")
Following the loss of the Gagarin, Discovery sought to use the natural crystal transmitter located on the planet Pahvo to locate cloaked Klingon vessels. However, their actions instead led the Pahvans to transmit a signal which attracted the attention of the Klingon flagship Sarcophagus. The crew of Discovery managed to board the Klingon ship and recover Admiral Cornwell from captivity, planting beacons to penetrate the cloaking field. Using the spore drive, Discovery was able to gather enough data to complete an algorithm which would counteract the cloaking technology, and then destroyed the Klingon ship. They intended to transmit this information to Starfleet, but did not get the chance. Attempting to use the spore drive to jump to Starbase 46, the Discovery instead found itself in a parallel universe. Unbeknownst to Starfleet or the Discovery crew, the commanding officer, Gabriel Lorca, did this intentionally, trapping the ship and its crew. (DIS: "Into the Forest I Go", "Despite Yourself")
Federation retreats and later engagements
At the same time, the ISS Discovery exchanged places with its counterpart, was attacked by Klingon forces, and was destroyed, leading Starfleet to believe that its Discovery had been lost. Without the cloak-penetrating algorithm or the vital asset that Discovery had become, the Federation was unable to hold back the Klingon forces. In addition, having lost another unifying leader, the Klingons were no longer acting as a single force – instead, the twenty-four Houses competed for dominance by seeing which House could destroy the most Federation assets. The attacks were indiscriminate, targeting hospitals, civilians, and food convoys. (DIS: "The War Without, The War Within")
At some point, the USS Hiawatha, a medical frigate, was shot down on its way to Starbase 36. It was believed destroyed, but in fact, had crashed on the surface of an interstellar asteroid in enemy space. Most of its war-wounded were able to evacuate in escape pods, though a few were too injured to be moved. Engineer Jett Reno volunteered to stay behind to care for them and did so for ten months and eleven days, until being rescued after the end of the war. (DIS: "Brother")
On stardate 4789.6, hypothermic charges were used against Kelfour VI, burning off the planet's atmosphere and instantly killing eleven thousand Federation civilians. Stardate 4851.5 saw the USS Saratoga tailed by a cloaked Klingon raider as it arrived at Starbase 22; the raider then self-destructed, destroying the starbase. Similar attacks against Starbase 12 and Starbase 19 in the following days ultimately resulted in the loss of a third of the Federation fleet. Further assaults on the same stardate wiped out research outposts on Nivalla, Septra, and Iridin. (DIS: "The War Without, The War Within")
Nine months after the destruction of the Sarcophagus, nearly twenty percent of former Federation space was occupied by the Klingons. At this point, Discovery returned to its own universe via the mycelial network, only arriving at that time due to having overshot their intended return trajectory. (DIS: "What's Past Is Prologue", "The War Without, The War Within")
Although Discovery's return allowed the distribution of the algorithm, it was too late to prevent an assault on Starbase 1 by forces of House D'Ghor, who eliminated the eighty thousand people aboard – including a significant part of Starfleet leadership – along with the three starships defending it. Discovery encountered the aftermath of the attack, with a House D'Ghor sigil decorating the fallen base.
With Klingon forces on the verge of striking Earth, and considering standard tactics to have failed, the Federation Council was forced to adopt a plan suggested by the stranded Terran emperor, Philippa Georgiou. As a result, it authorized Discovery to use its spore drive to jump into the subterranean caverns of Qo'noS in order to map the planet's defenses and infrastructure, ahead of an all-out assault on the planet, with the aim of forcing the Klingons to retreat from Federation space. The mirror Georgiou was disguised as her counterpart and placed in command of Discovery for the duration of the mission. (DIS: "The War Without, The War Within")
In reality, the mission was to detonate a hydro bomb within a volcano on Qo'noS. By the time Discovery's crew became aware of this, it was too late to prevent the bomb from being planted, but not to talk Cornwell out of the mission. Preventing Georgiou from detonating the device, Burnham gave the detonator to L'Rell, who used the threat of detonating the device to force the leaders of the Great Houses to accept her leadership role in the Empire. She ordered Klingon forces to halt their advance on the Federation, thus ending the war, and then began working to continue T'Kuvma's work of uniting the various Klingon factions. (DIS: "Will You Take My Hand?")
Following the war, Chancellor L'Rell ordered the mass production of a new class of warship, the D7-class battle cruiser, a gesture to strengthen the tenuous union of Great Houses. (DIS: "Point of Light")
The war between the Klingon Empire and the Federation went on to serve as a violent precursor of what was to come; in 2267, diplomatic relations between the two sides again collapsed, resulting in a brief conflict that included the planet of Organia. (TOS: "Errand of Mercy")
|Battles of the Federation-Klingon War (2256-57)|
|2256: Battle of the Binary Stars • Attack on Corvan II • Battle at Pahvo|
|2257: Scorching of Kelfour VI • Attack on Starbase 22 • Attack on Starbase 12 • Attack on Starbase 19 • Attack on Nivalla • Attack on Septra • Attack on Iridin • Capture of Starbase 1|
|Klingon War (2256-57) • Klingon War (2267) • Cardassian War • Galen border conflict • Tzenkethi War • Dominion cold war • Klingon War (2372-73) • Dominion War • Temporal Cold War • Klingon War (2340s-2366)|
|Federation War (2256-57) • Federation War (2267) • Klingon Civil War • Cardassian War • Dominion cold war • Federation War (2372-73) • Dominion War • Federation War (2340s-2366)|
Note on total casualties:
- The number of deaths is established as ten thousand in "Magic to Make the Sanest Man Go Mad", at least nine months before the end of the war. Later, the incident at Kelfour VI is said to have resulted in the death of eleven thousand civilians. Except for an earlier kill count, these are the only hard numbers given, although given the massive losses in the last nine months of the war the count might be substantially higher. Furthermore, after Starbase 1 is captured by the House of D'Ghor, eighty thousand life signs are found to be missing, suggesting these might have been killed, although alternatively they might have managed to evacuate.
The Battle of Caleb IV, including Kor commanding a D5 cruiser, is mentioned in the Star Trek: Deep Space Nine episode "Once More Unto the Breach" and was indirectly implied, by Kor, to have taken place when cloaking technology was a novelty in the Klingon fleet, which could place that engagement as part of this conflict.
A mission to Kim-Tara, for which Starfleet developed landing pods, was presumably also planned during the war, as Burnham's involvement as a test pilot puts the program between "Context Is for Kings" and "Will You Take My Hand?".
Recalling how this period of conflict had been depicted prior to being portrayed in DIS Season 1, Alex Kurtzman remarked, "The war with the Klingons, or the skirmishes, as they were called, with the Klingons, had been alluded to but never really expanded on. It's a chapter that has a lot of room for interpretation." ("Discovering Discovery: The Concepts and Casting of Star Trek: Discovery", DIS Season 1 DVD and Blu-ray special features)