DS9 Season 4 Episode 1+2 The Way of the Warrior

Plot

A Klingon armada led by General Martok arrives at Deep Space Nine; Martok tells Captain Sisko they are there to defend their Federation allies against potential invasion from the Dominion, a hostile empire led by the shape-shifting Changelings. As the Klingon soldiers spread throughout the station, they cause disruptions, and beat up the Cardassian tailor and former spy Garak. A Klingon ship detains Kasidy Yates's freighter to search the ship for Changelings. It takes a show of force from Sisko in the Defiant to convince the Klingon commander to back off; but the Klingons continue detaining ships just outside Deep Space Nine's jurisdiction.
Sisko sends for the sole Klingon in Starfleet, Lieutenant Commander Worf, to help deal with the Klingons. Although Worf is considering resigning from Starfleet, he investigates the Klingon fleet and discovers its true purpose: the Klingon Empire plans on attacking Cardassia. The Cardassian military government has been overthrown in favor of the civilian Detapa Council, but the Klingons believe that the coup was engineered by the Dominion as a way to take control of Cardassia. Sisko warns Martok that such an attack would jeopardize the Federation's alliance with the Klingons, but Martok orders the invasion to proceed. Sisko allows Garak to overhear news of the invasion; he in turn warns Cardassia. Once the Federation condemns the invasion, the Klingon Empire withdraws from the Khitomer Accords, ending its alliance with the Federation. Klingon chancellor Gowron invites Worf to join the invasion, but Worf refuses out of loyalty to Starfleet, jeopardizing his family's social status in the Klingon Empire.
Sisko convinces the Detapa Council to flee Cardassia and seek refuge at DS9, escorted by the Defiant. At the rendezvous point, the Cardassians are attacked by Klingon ships; the Defiant joins the battle, rescuing the Council. When the Defiant returns to DS9, Sisko informs Gowron that the council members are not Changelings, but Gowron orders his fleet to attack DS9 anyway. DS9 defends itself with newly upgraded weapons systems, destroying many Klingon ships; Klingon boarding parties beam into DS9, but the crew fights them off and regains control of the situation.
With Starfleet reinforcements on their way, the Klingon ships have no choice but to retreat. Over Martok's protest, Gowron calls off the invasion of Cardassia, but refuses to cede the outlying Cardassian planets the Klingons have already seized. Worf decides to remain in Starfleet and accepts the post of Strategic Operations Officer on DS9.