Federation-Klingon War (alternate timeline)
In an alternate timeline, the Federation-Klingon War was a major conflict between the Klingon Empire and the United Federation of Planets that started in the 2340s and lasted until 2366, following the deterioration of relations between the two powers.

Origins and overview
The alternate timeline in which the war was fought had been created when the  essentially disappeared into a temporal rift in 2344, while defending a Klingon outpost on Narendra III from a Romulan attack. This incident caused a breakdown in the negotiation of a peace treaty with the Klingons, and outright war ensued for twenty years.

By 2366, the war was going very badly for the Federation, far worse than most Federation citizens knew. Despite Starfleet emerging victorious from several battles – including one at Archer IV – forty billion lives had already been lost and Starfleet Command believed that defeat was inevitable within six months, leaving the Federation with no choice but to surrender. Half of their fleet had been lost, and all vessels were being pressed into service, regardless of age.

A tactical map of the war was displayed in the captain's ready room aboard the USS Enterprise-D, a  warship which was under the command of Captain Jean-Luc Picard and was capable of carrying over 6000 troops. Owing to the war, no children lived aboard the ship. For the Federation, the conflict also resulted in rationing of food due to concentrating power on defensive systems, and considerable advancement of deflector shield technology which increased the endurance of ships when in battle.

Prelude to conclusion

In 2366, on combat date 43625.2, the Enterprise-D encountered a radiation anomaly near Starbase 105, the same anomaly which the Enterprise-C had entered. The battle-damaged Enterprise-C then emerged from the anomaly, with 125 surviving officers on board. Shortly thereafter, Starfleet monitor stations detected a fleet of Klingon battle cruisers heading towards the two Enterprises, news which the stations relayed to the Enterprise-D. Although Captain Picard and Commander Riker from that ship arranged the salvaging of the Enterprise-C so Starfleet could make use of the newly returned vessel in the war, it was imperative for the Enterprise-D not to remain where it was for too long, due to the approaching Klingon fleet; Picard allowed nine hours for repairs. As the work progressed, Picard's personal confidante, the El-Aurian Guinan, who had somehow been able to intuitively sense the change in timeline, warned him that the war was not meant to be taking place and Picard realized the Enterprise-C's defense of the outpost was the pivotal point which would have prevented the conflict.

Enterprise-C commanding officer Captain Rachel Garrett ordered the repairs to her ship to be focused on the craft's photon torpedo launcher rather than the vessel's warp drive, due to the desperation of the Federation's war effort. It was expected that the pair of Enterprises would soon be engaged in battle, as Klingon warships had been detected in the same sector as them, by this time. Captain Garrett advised her crew to prepare to remain in the war, as the Federation was in desperate need of another ship to oppose the Klingons, but Picard planned for the Enterprise-C to return to the past from whence it came. He confidentially revealed to Garrett the truth of how badly the Federation was suffering in the conflict and how, by traveling back in time, the Enterprise-C could eliminate the war. These details convinced her to take the  vessel back through the rift in order to restore the intended timeline, in spite of the near-certain prospect of the ship's immediate destruction.

Unexpected attack
Shortly after Captain Garrett advised her crew that their vessel would be journeying back into the past, a Klingon Bird-of-Prey quickly decloaked and opened fire on the Enterprise-C. On that vessel's bridge, the assault came as an unexpected series of jolts. The Enterprise-C's shields were raised and the ship initiated evasive maneuvers with Gamma Sequence, preparing to return fire with phasers.

The Enterprise-D repeatedly fired phasers at the Bird-of-Prey, which was off its starboard bow. At least two of the Enterprise-D's phaser bolts struck the Klingon ship's starboard shielding; the first of these hit the starboard wing and the second battered against the bow. Concentrating fire on the Enterprise-C, the Klingon ship fired a disruptor volley at it.

{{bginfo|Although a view of firstly the Enterprise-D firing two phaser bursts at the Bird-of-Prey and then the Klingon craft firing a pair of disruptor bolts at the Enterprise-C was included in the episode, these actions weren't in the installment's final draft script. }}

The Enterprise-C retaliated with phaser fire. On the bridge, Captain Garrett ordered the loading of her ship's torpedo bays but, moments later, she was hit with debris and fell to the deck, dead.

The Enterprise-D returned fire on the Bird-of-Prey with three phaser shots which again hit the Klingon ship's starboard shields. The Bird-of-Prey next re-cloaked and Captain Garrett's death was reported to Captain Picard and the Enterprise-D. Its Ambassador-class predecessor had sustained moderate damage in the fracas and a piece of shrapnel had become lodged in Garrett's forehead.

Interim
In the aftermath of the battle, Lieutenant Richard Castillo, formerly the helmsman of the Enterprise-C and now the only surviving member of the vessel's senior staff, volunteered to take command of the ship. The Enterprise-D detected additional instability in the temporal rift, which was possibly a result of the recent battle. Though there was no sign of other Klingon ships in the vicinity, it was now vital for the Enterprise-D to depart the area, because the vessel's coordinates had clearly been sent to the Klingon Command. Final preparations were made for the Enterprise-C to get under way, with Castillo in command and its tactical station manned by  Lieutenant Tasha Yar, a tactical officer who voluntarily transferred from the Enterprise-D.

The Galaxy-class warship's long-range scanners meanwhile picked up numerous Klingon battle cruisers on an intercept course. Consequently, the Enterprise-D began to slowly head towards them.

As the Klingon ships approached, the Enterprise-D was able to distinguish them as a trio of confidently uncloaked  Birds-of-Prey.

Readying his ship to return to battle conditions, Captain Picard orated a ship-wide message in which he stated that, though the Enterprise-D could obviously outrun the Klingon fleet, the crew had to ensure the Enterprise-C's protection until it entered the time rift. Picard also gave encouragement to his troops; the last engagement was about to begin.

The final battle

As the Klingon warships proceeded steadily nearer the Enterprise-D at low velocity, they were arranged with two ships following a lead ship which was at the center forward position and opened fire with a series of twinned disruptor bolts. Ceaselessly continuing the approach, the starboard Bird-of-Prey soon joined in with the onslaught, firing similar blasts.

Aboard the Enterprise-D, Captain Picard then set a course change, which he soon corrected. After the craft detected that one of the Klingon warships was heading for the Enterprise-C, Picard ordered the Galaxy-class vessel be kept within 200 kilometers of the departing Starfleet craft.

The Enterprise-D was now beginning to suffer considerable stress. Damage control teams were required on and sent to deck fourteen, the ship's starboard power coupling was no longer working, and there was an imminent danger of losing antimatter containment. The Klingon warships were concurrently flanking the Galaxy-class ship, trying to draw it away from its Starfleet kin.

On Captain Picard's instruction, the Enterprise-D kept its course and released continual bursts from all its phasers, targeting multiple directions. One of the phaser streaks punched through the shields surrounding one of the Klingon vessels and resulted in its destruction; the defeated craft erupted in a huge explosion, through which another Klingon ship flew.

Four massive jolts were then registered on the bridge of the Enterprise-D, a crewman being thrown from his post, which sparked, during the second thump.

Owing to the coolant leak, a warp core breach was unavoidable and Chief Engineer Geordi La Forge estimated a duration of two minutes until it happened. Following an additional explosion, crew members began to evacuate Engineering. The ship shuddered even more. Fortunately, the Enterprise-C was merely fifty-two seconds away from entering the time rift. Captain Picard instructed all of the Enterprise-D's remaining power to be diverted to the defense systems. The forward phaser banks were no longer responding, and, on the bridge, another eruption threw Commander Riker from the tactical station to the deck, killing him with a bloody gash at his neck.

A Klingon commander contacted the Enterprise-D, demanding that its crew surrender and prepare to be boarded. These orders were followed by repeated pounding of the craft.

Softly but defiantly, Captain Picard answered, "That will be the day." He then jumped over the wooden handrail to assume the controls at the tactical console.

Picard fired a phaser streak along the underside of a passing Klingon craft, but, as flames started to engulf the bridge of the Enterprise-D, the ship was pummelled by the two remaining Klingon vessels, which closed in on the Enterprise-D. Moments later, the Enterprise-C managed to finally enter the rift, eradicating the war from the timeline by returning to the craft's own time. ()

Aftereffects
Affected by the events of the alternate timeline, Guinan found herself curious about Tasha Yar and asked Geordi about her. One other remnant of the timeline survived: the alternate Tasha Yar, who had journeyed aboard the Enterprise-C into the prime timeline. ()

Background information
 Origins 
In a story which provided some of the roots for "Yesterday's Enterprise", the origins of the Federation-Klingon war were evident in a timeline inspired by the mirror universe, from .  Eric A. Stillwell and Trent Christopher Ganino, the writers of this influential  storyline, conceived an alternate timeline in which the Vulcans had turned violent – due to Surak having been killed in the past – and joined forces with the Romulans in a super empire that wiped out the Klingons and turned on the Federation. () In fact, the Vulcan-Romulan Empire was on the verge of destroying the Federation at the time the story's events in the alternate timeline were set. (Conversations at Warp Speed, "Eric A. Stillwell: Star Trek Changed My Life'") The Romulans and Vulcans had joined forces several centuries beforehand and the devastating war had killed billions of people. (The Making of Yesterday's Enterprise, p. 30) Stillwell recalled, "The Vulcan-Romulan Empire had conspired to destroy the Federation by seeking to capture the Guardian planet and use [...] [the Guardian of Forever] to their own nefarious purposes [....] In the altered universe, the Guardian planet becomes the last stand for the battleship Enterprise in an effort to prevent the destruction of the Federation."  The Enterprise-D was ambushed in orbit of the Guardian Planet by several Romulan-Vulcan Warbirds which suddenly uncloaked, surrounding the Enterprise. The enemy attackers pounded and nearly destroyed the Federation vessel, but then the timeline was reset. (The Making of Yesterday's Enterprise, p. 31)

It was Ronald D. Moore who devised the idea that the opposition faced by the Federation in the war they were losing would be the Klingon Empire. ("Yesterday's Enterprise" audio commentary, TNG Season 3 Blu-ray) "Of course, the whole thing with the Romulans and Klingons got switched round," remembered Eric A. Stillwell, "because we thought it would be cool now if the Klingons were the ones who got pissed off." (Conversations at Warp Speed, "Eric A. Stillwell: Star Trek Changed My Life'") The Klingons were also in a spec script which was submitted by Trent Christopher Ganino and which, merged with the above-detailed plot, served as the basis for the storyline of "Yesterday's Enterprise". In the spec script, there was no mention of an alternate timeline, and history had recorded that many Klingon lives had been saved in an incident that involved the Enterprise-C and helped forge the peace treaty between the Federation and the Klingons. (The Making of Yesterday's Enterprise, pp. 26, 33 & 34)

A story outline about the amalgamated plot featured an alternate timeline in which the Klingons and the Federation never became allies and history recorded that the chance for peace was destroyed when, according to erroneous report, the Enterprise-C attacked a Klingon outpost. After the Enterprise-C traveled twenty-two years into the future, the Enterprise-D picked up a sensor reading of something which was invisible, possibly the time anomaly returning, but turned out to be a cloaked Klingon Bird-of-Prey on attack course. The Enterprise-D went to red alert status and raised shields. Captain Garrett was aboard the Enterprise-D, and, though she wanted to return to her own vessel, it was now too late for her to do so. At Riker's suggestion, the Enterprise-D began to move to a position where it could shield the Enterprise-C, coming between it and the Klingon craft. While the Enterprise-D was in transit, Picard sent an unanswered hail to the Klingons, and, moments later, the Enterprise-D was fired upon, after which it took evasive action. The ship then finally started to protect the Enterprise-C, much to the puzzlement of the Klingon Captain (who was suggested as perhaps being Worf), because the attack on the Klingon outpost had killed his parents. After Picard refused to turn over the Enterprise-C, the battle resumed, killing Garrett on the Enterprise-D's bridge. Afterwards, Picard managed to talk the Klingons into a temporary cease fire, as they seemed to be momentarily satisfied by Garrett's death. The timeline was restored shortly thereafter, saving the lives which would have been lost if the Klingons and Federation had remained adversaries. (The Making of Yesterday's Enterprise, pp. 40-44)

In a revised story outline written by Eric A. Stillwell and Trent Christopher Ganino, Picard reported in his captain's log, following the arrival of the Enterprise-C from the past to the alternate timeline, about potential aggression between the two powers. He admitted, "My only fear is that the reappearance of this ship could trigger renewed hostilities between the Federation and its long-time enemies... the Klingons." Since the history of the alternate timeline now documented that the Enterprise-C had been lost in space after allowing the destruction of a Klingon outpost, the Klingons had continually refused an alliance with the Federation, citing the incident. The danger of the Klingons noticing two Federation vessels so near their territory put pressure on the repairs to the Enterprise-C, which were consequently hurried. The Enterprise-D activated red alert during a conversation, in one of the ship's corridors, between Picard, Guinan, Garrett, and Enterprise-C Science Officer Donovan (the latter of whom later evolved into Lieutenant Castillo, as the story gradually developed). Though Garrett and Donovan wanted to return to their own vessel, nobody could leave the Enterprise-D. After the two captains went to the bridge, Riker notified them a Klingon battle cruiser was approaching with weapons armed. Picard, motivated by Garrett voicing concern for her ship, ordered the Enterprise-D to come between its Starfleet forerunner and the enemy craft. After Picard hailed the Klingons, they demanded he surrender the Enterprise-C to them, but Picard refused. A battle ensued, in which Garrett again died on the Enterprise-D's bridge. Picard was forced to destroy the Klingon ship, prior to the timeline being reset. (The Making of Yesterday's Enterprise, pp. 47-52)

So far, Eric A. Stillwell and Trent Christopher Ganino had deliberately avoided making the war particularly bleak, out of fear of writing a story so expensive to produce it would never be filmed. (The Making of Yesterday's Enterprise, p. 59) Ron Moore offered, "Yes, the Federation was at war, but it [the Enterprise-D] didn't feel like a warship." ("Inside the Writer's Room", TNG Season 3 Blu-ray) Besides, Moore, who Michael Piller gave the responsibility of further developing the story, felt the version handed to him lacked any obvious proof of a war with the Klingons. "It needed to be clear that the Federation wasn't just fighting a war, they were losing a war," he remarked. (Star Trek: The Official Starships Collection, issue 46, p. 16) Hence, Moore originated the conditions of the war, such as the grimness and militarism of the alternate timeline. He hit upon these notions while writing a couple of story outlines for the episode. (Cinefantastique, Vol. 21, No. 2, p. 46) For instance, in a revised story outline he wrote, Moore stated, "War with the Klingons has been a recurrent problem for the past couple of decades and there is no end in sight [....] This alternate view of the future is much grimmer than the universe we're used to seeing. The universe is a much darker and harsher place than before." (The Making of Yesterday's Enterprise, p. 56)

In his version of the story, Ron Moore imagined a sudden Klingon attack disrupting the study of the Enterprise-C. A brief skirmish then ensued, in which the Enterprise-D was slightly damaged whereas the Enterprise-C was heavily damaged. The battle also disturbed the time rift, so it slowly began contracting in on itself and was to disappear in several hours. Captain Garrett was also killed on the bridge of her own ship, this time, along with most of her senior staff, when the room was heavily damaged in the conflict. No description was given as to how this skirmish ended. After a brief interim, approaching Klingon ships were detected. They arrived, ready for battle, just as the Enterprise-C re-entered the temporal anomaly, resetting the timeline. (The Making of Yesterday's Enterprise, pp. 56-57)

In a beat sheet, Ron Moore made implicit what he had merely implied before; his version of the story featured the idea that the Federation and the Klingons had been at war for twenty-two years. After the wounded survivors of the Enterprise-C began receiving assistance from the Enterprise-D, Data reminded Picard of there possibly being Klingon war vessels in the area. The possibility of a Klingon attack meant it was wise not to stay in the vicinity for too long and concerned Picard, who allowed five hours for repairs to the Enterprise-C. The same beat sheet specified that the war had partly started because the Klingons considered Starfleet to be cowards for not coming to their rescue. At one point in the alternate timeline detailed in the beat sheet, Picard and Riker had a discussion which involved them reflecting on how the war had prevented what they had hoped to accomplish when they joined Starfleet. Later, instead of only a single Klingon Bird-of-Prey, two Klingon battle cruisers appeared, attacking both Enterprises. Just as in the episode's final version, they repelled the attack, despite the conflict costing Captain Garrett's life, and long-range scanners thereafter picked up a fleet of approaching Klingon ships prior to a final battle. The beat sheet's description of the last fray read, "The Klingons attack. Our task is to hold them off long enough for the Enterprise-C to get back into the rift. We're hopelessly outnumbered. The battle rages and we fight to the last man – Picard, standing alone on the burning bridge and firing the phasers manually. Just as the Klingons are about to deliver the coup de grace [the timeline is reset]." (The Making of Yesterday's Enterprise, pp. 61-64)

 Analysis and production aspects 

Ronald D. Moore believed the bleakness of the war was realistic. He stated, "I've heard from time to time, I wish you'd do some war stories, but this is the reality of war. It's not a pretty place. But it was a lot of fun to watch that ship [the alternate Enterprise-D] move." (Cinefantastique, Vol. 21, No. 2, p. 46) He clarified, "I just really like the fact that the Federation is losing the war and they're doomed [....] There was something just really interesting about that idea." Additionally, Moore reckoned that, if the writing staff had had sufficient time, the effects of the war on TNG's main characters could have easily been explored in a two-part version of the story. ( audio commentary, TNG Season 3 Blu-ray)

The alternate timeline and the war therein hinted at the idea that the utopian future and Starfleet code of ethics usually presented in Star Trek could easily crumble when faced with the opposition of a serious external threat. Ira Steven Behr not only co-wrote "Yesterday's Enterprise" but was also deeply skeptical that Starfleet's ideals could survive any such hostility. (BFI TV Classics - Star Trek, p. 102) He said of the episode, "It was a chance to see the crew fighting in a big, nasty battle." ()

The war was also a hypothetical analogy for what the world might be like if Earth's real Cold War had turned hot. This was apt because, at the conceptual genesis of relations between the Klingons and Federation as well as for a long time thereafter, the Klingons had been metaphorical for the Russians and the United Federation of Planets had represented the United States of America. (Star Trek and History, "Part Two: Kirk and Spock Take on Earth History", "Chapter 5 – The Final Reflection?: A Mirrored Empire?", "Too Klingon to Be Human")

In David Carson's opinion, the final events of the war – those depicted in "Yesterday's Enterprise" – had an even more stressful impact on the characters than they were used to. "There is an intensity and an urgency about everything that is going on that is, sort of, beyond the fact that they're at war," he commented. "I mean, they've been at war for twenty years. You would think that they're [...] somewhat used to it. However, the story itself instilled in the actors a great sense of urgency and tension. It was as if every day was a new day of war for them. They're also able to play in a different way [....] The battle scenes were greatly helped by the multi-level appearance of the new battle bridge [on the Enterprise-D].''" ( audio commentary, TNG Season 3 Blu-ray)

The visual effects incorporated into the depiction of the war were due to Visual Effects Supervisor Dan Curry. The fatal injuries which killed Captain Garrett and Commander Riker were represented by Make-Up Supervisor Michael Westmore. ( audio commentary, TNG Season 3 Blu-ray)

Footage for the battle, including the Klingon Bird-of-Prey studio model, was being shot on . It was shot at 30 fps. 

nl:Federatie-Klingon oorlog (alternatieve tijdlijn)
Federation-Klingon War (2340s-2366)
Federation-Klingon War (2340s-2366)
Federation-Klingon War (2340s-2366)