The attack on Bortas was the first battle of the Klingon Civil War. It was an unsuccessful attempt by forces loyal to the House of Duras to target Gowron, the new Chancellor of the Klingon High Council, after several members of the Klingon High Council rejected Gowron as leader and joined the Duras family in challenging him.
Prelude
Duras, the head of the House had been Gowron's main rival to succeed the recently deceased chancellor, K'mpec, months earlier. Before the successor could be determined, however, Duras was killed in personal combat by Worf, claim the Right of Vengeance for having killed K'Ehleyr. At the same time, a molecular decay detonator, a Romulan device used to set off a bomb during the Sonchi ceremony was traced back to one of Duras' men, exposing Duras as a Romulan collaborator. (TNG: "Reunion")
Although Duras had died in disgrace, his family's political power and influence spared them from sharing in that disgrace, as a Klingon family normally would. Duras' sister Lursa and B'Etor assumed de facto leadership of the House. Although, as women, they were not permitted to lead the High Council, Gowron was certain they still planned to usurp his leadership.
After relaying these suspicions to the Arbiter of Succession, Captain Jean-Luc Picard, Gowron was confronted by Worf. Although Word had killed Duras, Gowron initially refused to speak to Worf, as he admitted guilt before the High Council in 2366 and undergone discommendation for his father Mogh supposedly having collaborated with the Romulan Star Empire in the Khitomer Massacre years earlier. Worf, however, revealed that he had been forced to admit guilt as part of a cover-up to protect Duras, whose own father Ja'rod had been the true collaborator. Worf asked Gowron to restore his family's good name.
Gowron was shocked and outraged that the Council had been complicit in this cover-up. He expressed gratitude to Worf for having Killed Duras, but was unwilling to alienate the council by exposing their involvement in this cover-up. (TNG: "Sins of the Father", "Redemption")
Gowron's suspicions were confirmed when the sisters presented Toras an illegitimate young adult son of Duras, whom the sisters apparently intended to use as a puppet leader they could rule through.
Ultimately, however, Picard, as Arbiter, recognized Toral as Duras' son, but concluded that Toral earned no honor or distinctions in his own right and ruled that Duras' claim to leadership had died with him. Toral accused the council of allowing the United Federation of Planets to determine Klingon destiny. He urged them to follow him instead, then left, with most of the council joining him.
Soon after, Worf again approached Gowron aboard the IKS Bortas, now offering the support of four squadrons loyal to Worf's younger brother Kurn, in exchange for the restoration of his family honor. Gowron was unswayed, as the Duras family controlled most of Klingon Defense Force fleet. Instead, he urged Worf to persuade Picard to convince Starfleet to provide assistance. Worf was unwilling to do this, however the argument was interrupted by a sudden attack
Conflict
A Klingon Bird-of-Prey loyal to the Duras had uncloaked and opened fire on the Bortas, and was quickly joined by a second Bird-of-Prey. Although larger and more powerful than a single Bird-of-Prey, the Bortas was now outnumbered two to one and suffered heavy damage, losing disruptor banks and diminished shields in their port quarter. Gowron ordered a distress call to any loyal ships.
The USS Enterprise-D was orbiting nearby, but Picard instead ordered the Enterprise to move clear of the combat area. Although he had served as Gowron's Arbiter, and Worf was a member of his crew, Picard was unwilling to drag the Federation into a Klingon civil war by going to the aid of the Bortas.
Aboard the Bortas, weapons were quickly restored and Worf assumed the weapons post. Gowron gave the order to return fire, but Worf pointed out that the enemy ships would detect the weapons lock. Instead, he suggested allowing them to believe the Bortas was helpless, luring them in, at which point Worf would aim and fire disruptors manually when they lowered shields to beam over a boarding party.
The Birds-of-Prey moved in, and, as Worf predicted, lowered shields. Worf fired, successfully destroying one ship, but was unable to fire on the other before it raised its shields. The Bird-of-Prey resumed firing, disabling weapons and causing its shields to fail.
Just when the Bortas appeared helpless, a third Bird-of-Prey the IKS Hegh'ta appeared, under the command of Kurn. Pledging loyalty to Gowron, Kurn fired on the first Bird-of-Prey, forcing it to retreat.
Aftermath
Impressed with Worf and Kurn's service during the battle, Gowron, after being installed as chancellor, finally agreed to restore their family honor. There was little time for celebration, however, as the Duras' forces still possessed greater numbers in the bloody civil war that followed. (TNG: "Redemption")
After Picard further refused to involve Starfleet in the conflict, Worf temporarily resigned his commission as a Starfleet, serving as tactical officer aboard the Bortas and later first officer of the Hegh'ta. (TNG: "Redemption", "Redemption II")