A Klingon Bird of Prey fires on Chancellor Gowron's flagship
The Klingon Civil War began over a disagreement in Klingon leadership after the death of Chancellor K'mpec. The chosen Arbiter of Succession, Federation Captain Jean-Luc Picard, weighed the two challengers to succeed K'mpec: the political outsider Gowron or the politically connected Duras.
Duras' claim to the chancellorship effectively ended when he was killed in honorable combat by Worf. His house and his faction discredited, the way was cleared for Gowron to assume leadership of the Council.
Although Duras had died, his claim to the chancellorship did not die with him. His House, now lead by his sisters Lursa and B'Etor introduced Duras' illegitimate child Toral as Gowron's challenger. As the Arbiter of Succession, Captain Picard eventually rejected Toral's claim, arguing that Toral had claimed no honor for himself, having fought no battles for the Empire.
The House of Duras claimed Federation meddling in Klingon internal 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 suffered extensive damage. He was eventually saved by the forces of Captain Kurn and the IKS Hegh'ta. Persuaded by his older brother Worf, Kurn brought the support of three full fleet squadrons with him. With this support, Gowron officially became the Chancellor of the Klingon Empire.
As official leader, Gowron found his allies thin. The United Federation of Planets decided not to intervene in what was called "by definition an internal Klingon matter." Gowron's forces were seriously outnumbered by the Duras forces. Over the next two weeks, Gowron's forces suffered three major defeats, most notably at the Battle of Mempa.
Captain Picard met with Starfleet Command to propose that if outside forces were affecting the Klingon Civil War, it would very much become a Federation issue. Once evidence was found of Romulan complicity with the Duras faction, Picard led a blockade of 23 starships into Klingon territory to deploy a tachyon detection net, which would detect cloaked Romulan ships. The Romulan viewed it as a hostile act and attempted to circumvent the detection net. Their ploy was foiled and the Romulan ships detected by the USS Sutherland, with Lt. Commander Data commanding. Once the Romulan connection to the House of Duras was exposed, Klingon support for the House of Duras fell apart. Gowron was able to lead an offensive that ultimately provided 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.