Star Trek Deep Space Nine: The Way of the Warrior

A fleet of Klingon ships is stationed around Deep Space Nine, and the Promenade is swarming with Klingons. However, the warriors aren't their usual antagonistic selves. General Martok, who leads the force, tells Sisko they have come to help the Federation fight the Dominion. Sisko finds this odd, since the Federation neither requested nor expected help. Garak and Odo then have a run-in with Drex, a young Klingon, who later attacks Garak for no apparent reason. 
When a freighter is stopped by a Klingon Bird-of-Prey, Sisko confronts its commander, Kaybok. Under orders from the Klingon High Council to assist the Federation alliance, he insists the freighter must be inspected for Changelings. But Sisko, aboard the Defiant, forces him to let the vessel pass. Martok later has Kaybok killed for not carrying out his orders. Since Martok will not explain their motives, Sisko summons Lieutenant Commander Worf, formerly of the U.S.S. Enterprise, hoping the Klingon officer can determine their plans. 
Worf starts a fight with Drex, Martok's son, in order to get the general's attention, but Martok won't reveal why the Klingons are on Deep Space Nine. Worf's allies are equally closed-mouthed, but he soon learns the truth from a Klingon officer indebted to his family. Realizing he is betraying his own people, Worf reluctantly informs Sisko that the Klingons are planning a massive invasion of Cardassia. Apparently, a coup on that world has left a civilian regime in power, one which the Klingons insist is backed by the Dominion. But, with no proof to support this belief, Martok is asked by Sisko to call off the invasion. However, the Klingon general instead leads his fleet on a course into Cardassian space. 
The Federation orders Sisko to remain neutral until word is heard from the Klingon leader, Gowron, who Worf believes is using the Dominion takeover idea as an excuse to reclaim their race's warrior heritage. Garak is allowed to learn about the dire situation, after which he tips off Gul Dukat on their homeworld, allowing the Cardassians a chance to defend themselves. The Federation condemns the invasion, at which point the Klingon Empire withdraws from their peace treaty. Gowron then comes to Deep Space Nine and offers Worf the chance to redeem himself by joining his people in the assault on Cardassia.
Torn between his duty and his people, Worf declines, prompting Gowron to excommunicate Worf's entire family. Worf then attempts to resign from Starfleet, but Sisko refuses to accept his resignation at this critical time. Sisko decides it's time to speak with Dukat, now part of the new government, and tells him that he will help escort the ruling council out of the war zone, even though, as Worf points out, he might actually be helping the Dominion. Sisko, Worf, Dax, and Bashir take the Defiant to a rendezvous point, where Dukat's ship is under heavy fire. The Defiant decloaks and prepares to attack the Klingons, ending two decades of peace. 
While engaged in battle with the Klingon vessels, Sisko and his officers beam aboard the Cardassians, losing the Defiant's cloaking device in the process. They head back to the station with two Klingon ships in pursuit, while Bashir tests the blood of the council members to make sure they aren't Changelings. When they finally reach Deep Space Nine, several dozen Klingon ships arrive ready to attack. 
Sisko prepares the station for the imminent onslaught. Martok demands for him to surrender the council members, but is told that the Cardassians are not Changelings. However, Gowron says that this doesn't matter because the Alpha Quadrant will be safer with Klingons in control of Cardassia. Sisko refuses his demands and warns Gowron that several weapons upgrades have been made to the station. Gowron dismisses the threat and the Klingons attack. 
Deep Space Nine opens fire, inflicting heavy damage on the Klingon fleet, but Gowron doesn't give up, and Klingon boarding parties begin to overrun the station, fighting in Ops and on the Promenade. The attackers are contained, then several Starfleet ships appear on the scanners. Sisko points out to Gowron that splitting the Federation, the Klingons, and the Cardassians is probably just what the Dominion wants. Gowron sees the logic and opts to stand down, but tells Sisko that he will not forgive or forget what has happened. Afterward, Sisko convinces Worf to remain as his new Strategic Operations Officer. But Dax interrupts the celebration with some grim news. The Klingons are refusing to give up the Cardassian colonies they've seized, meaning they are there to stay.