The Klingon Civil War was a conflict fought in early 2368 over the right to choose the next Chancellor of the Klingon High Council and de facto leader of the Klingon Empire. The war was fought between forces loyal to Gowron, the legally appointed candidate, and the supporters of the House of Duras led by Lursa and B'Etor. The war was a decisive victory for Gowron, but it was achieved only through indirect assistance from the United Federation of Planets in exposing covert Romulan assistance to the forces of the Duras family.
The conflict had its roots in the reign of K'mpec, the longest-lived chancellor of the Empire. Although K'mpec ruled with an iron hand and managed to keep peace within the Empire, his influence began to wane as he grew older, and two opposing factions began to develop. The first faction, led by Gowron, favored the continuing alliance with the Federation, while the second faction, led by Duras, sought to establish a more independent and aggressive policy for the Empire. However, Duras had support from a large majority of the High Council, while Gowron's support came largely from political outsiders and military personnel.
Tensions came to a head in early 2367 with the sudden revelation that K'mpec was dying. He had been slowly poisoned by Veridium Six that had been introduced into his bloodwine. Because the killer had not shown his face to his victim, Klingon tradition considered the act to be without honor. Recognizing the corruption that had taken hold in the Empire, K'mpec decided that he could not trust any Klingon with the task of choosing the next leader of the Empire. Therefore, he took the unheard-of action of selecting Jean-Luc Picard, a Human Starfleet officer, as the Arbiter of Succession.
Picard initially refused K'mpec's request; however, K'mpec argued that a Klingon civil war would inevitably involve neighboring powers — the Tholian Assembly, the Ferengi Alliance, and ultimately the Romulan Star Empire and the Federation itself. K'mpec pointed out that whoever had so dishonorably killed him — either Gowron or Duras — could not be allowed to rule the Empire. Concerned by the prospect of a massive interstellar war, Picard agreed to arbitrate the dispute.
The process of selecting a new leader for the Empire was almost immediately disrupted, however, when a bomb exploded in the middle of the Sonchi ceremony announcing K'mpec's death. One retainer each from Gowron's and Duras's delegations was killed in the blast.
Picard, wary of K'mpec's warnings concerning the dishonorable actions of either Duras or Gowron, chose to proceed carefully. He revived the ancient ritual of the ja'chuq, in which each contender tried to prove his worthiness to lead the Council, and delayed the final decision long enough to allow the Enterprise crew to investigate the bombing.
What the Enterprise officers found was disturbing: the bomb had used a molecular decay detonator, a type of device used only by the Romulans. Given the past history between the House of Duras and the Romulans — Duras had previously been involved in a coverup of his father Ja'rod's responsibility for the Khitomer Massacre some 20 years before (c.f. TNG: "Sins of the Father") — there was now evidence pointing towards one of the contenders.
The proceedings were abruptly ended, however, by a fight between Duras and Worf. Duras had killed K'ehleyr, the Federation ambassador to the Klingon Empire, for investigating into the matter of the Khitomer Massacre and uncovering Duras's father's complicity. Worf, as K'ehleyr's mate, claimed the right of vengeance against Duras, and killed Duras in honorable combat. Although Worf was officially persona non grata within the Empire, the High Council considered the matter closed. With Duras killed and his faction discredited, the way was cleared for Gowron to assume leadership of the Council.
Although Duras died in disgrace, his family did not inherit that disgrace. The corruption of Duras had spread to much of the rest of the High Council, who stood ready to support the House of Duras despite the traditional dishonor of its now-dead leader. Lursa and B'Etor, the sisters of Duras, assumed the leadership of the House, and began gathering support to overpower Gowron. They also secured the allegiance of three prominent fleet commanders, swelling their ranks for a possible military challenge to Gowron's leadership.
Gowron requested that Picard, as the Arbiter of Succession, ensure his installation as Chancellor. Picard, however, would not step outside the boundaries of Klingon law, and refused Gowron's request for assistance. He would only assure Gowron that he would respond to any challenges according to Klingon law.
The sisters' power play, when it came, took an unexpected form: they introduced to the Council Toral, a previously-unknown illegitimate son and heir to Duras. Toral challenged Gowron for the leadership of the Council, and the Council, controlled almost entirely by supporters of the Duras family, backed his challenge.
Picard was thus put in an extremely untenable position — if he accepted Toral's challenge, Gowron would quickly be killed and Toral installed as leader, but if he rejected Toral's challenge, Picard's supposed "Federation meddling" would be used as a rallying cry to gain support for the Duras family's cause. Ultimately, Picard chose to reject Toral's claim, arguing that Toral had claimed no honor for himself, fought no battles for the Empire.
Incensed, Lursa and B'Etor protested Picard's dictating of Klingon affairs, and led most of the High Council in walking out of the Great Hall. Gowron was left with few allies and practically no support.
The war finally broke out with a brazen attack on Gowron's flagship, the IKS Bortas. Not expecting an attack so quickly, Gowron was taken by surprise and his ship was nearly destroyed before the attackers were driven off by Captain Kurn and the IKS Hegh'ta. Kurn, an outspoken opponent of both Gowron and Duras, had been pursuaded to support Gowron by his elder brother, Worf. Kurn brought the support of three full fleet squadrons with him, which gave Gowron a fighting chance to resist his enemies. Emboldened by this support, Gowron chose to go through with his installation as Chancellor of the Klingon Empire.
As the official leader of the Empire, Gowron requested that the Federation send support in fighting the Duras family's forces under the terms of the Treaty of Alliance. However, Jean-Luc Picard, acting as representative for the Federation, refused to commit Federation resources, calling the conflict "by definition an internal Klingon matter."
Although he had some support to mount resistance, Gowron was seriously outnumbered, and the Duras forces were gathering a large fleet near Beta Thoridar. Over the next two weeks, Gowron's forces suffered three major defeats, most notably at the Battle of Mempa.
However, although the Battle of Mempa was a major defeat for Gowron, it was also a notable battle because it could have resulted in a complete rout. General Martok first became a revered figure in the Klingon military for his heroic leadership on that day (c.f. DS9: "Apocalypse Rising"). Additionally, Captain Kurn used a brilliant and unorthodox maneuver of ordering the Hegh'ta to use a solar flare to destroy two pursuing warships.
In the Federation, many eyes were closely watching the reports coming from the battlefields. Although the Prime Directive dictated that the Federation remain neutral in the war, Captain Picard correctly pointed out that if other powers — such as the Romulans — were involved in the conflict, then it would very much be a Federation concern. Picard argued that the unbroken string of Duras victories indicated that they were receiving outside support. Indeed, Picard pointed out that the Romulans had been trying to destabilize the Federation-Klingon alliance for the past twenty years, and it was almost unthinkable that they would remain uninvolved at such a crucial crossroads in the affairs of the Empire.
In a proposal to Fleet Admiral Shanthi, Picard suggested that Starfleet blockade the Klingon-Romulan border and prevent any ships from sending any further (still-hypothetical) support to the Duras family. Using a new technique called a tachyon detection grid, even cloaked ships would be detectable to Starfleet sensors.
Shanthi and the Federation Council authorized Picard's plan, and the USS Enterprise-D led a 23-ship fleet into Klingon territory to deploy the tachyon net. This action drew immediate protest from the Romulan side of the border, as Commander Sela demanded that the Federation fleet withdraw or be considered hostile and a potential invasion force.
In order to provoke the Romulans into attempting to run the blockade, Picard suggested that Gowron launch a full-scale assault on the Duras forces in the hopes of forcing the Duras to call for help from their secret allies. The Romulans took the bait, and despite an attempt to evade detection (that was defeated thanks to the initiative of Lt. Commander Data aboard the USS Sutherland, the Romulan ships were caught in the grid.
With the connection to the Romulans exposed, they retreated back to their own territory rather than risk a fight with both the Klingons and the Federation. Support fell away from the Duras family, and Gowron's offensive led to a complete victory.
Following his victory, Gowron proceeded to establish strict control over the Empire. He appointed an entirely new High Council membership, including Kurn and several other key allies. Although Gowron's position was threatened by the appearance a clone of Kahless the Unforgettable in 2369, Gowron was generally treated as the uncontested leader of the Empire until 2375.
Although Lursa and B'Etor escaped, their political influence was eliminated. Over the next several years, they attempted to regain their former status by attempting to raise funds with which to hire mercenary armies, or trying to obtain superweapons with which to blackmail the High Council. Lursa and B'Etor were finally killed in 2371 when their Bird-of-Prey was destroyed by the USS Enterprise-D in orbit of Veridian III.