DS9 "Rules of Engagement":
T'LARA: This hearing will come to order. We are here to consider the request of the Klingon Empire that Lieutenant Commander Worf be EXTRADITED for trial on charges of murder. Advocate Ch'Pok, you may present your charges.
CH'POK: The Klingon Empire makes the following allegations against Lieutenant Commander Worf. That on Stardate 49648, while commanding the Starship Defiant, he knowingly fired upon and destroyed a Klingon civilian transport ship near the Pentath system. That as a result of that action, four hundred forty one Klingon civilians were killed. It is my intention to prove that Mister Worf was grossly negligent in his command of the Defiant. That his lust for combat overrode his good judgement. I ask only that he be returned to us to face the judgment of his own people. Thank you.
SISKO: The Advocate neglected to mention in his opening statement that at the time in question, the Defiant was under attack by two Klingon warships. This was a combat situation with hundreds of lives at stake. It was at that moment, when suddenly the transport ship decloaked in front of the Defiant. Worf gave the order to fire, not because he was reckless or negligent, but because he believed he was firing on a warship. We intend to show that the destruction of the transport was a tragic, but unavoidable, accident.
T'LARA: I will hear formal evidence beginning tomorrow afternoon at fifteen hundred hours.
DS9 "The Way of the Warrior":
SISKO: They've decided to condemn the Klingon invasion. In response, Gowron has expelled all Federation citizens from the Klingon Empire and recalled his ambassadors from the Federation.
KIRA: You're saying he cut off diplomatic relations?
SISKO: He's done more than that. The Klingons have withdrawn from the Khitomer Accords. The peace treaty between the Federation and the Klingon Empire has ended.
It wasn't until a year later that the Khitomer accords were reinstated. The Episode "Rules of Engagement" occurred during the time the Khitomer accords were not in effect. Further as Kira states there are no more diplomatic relations, Federation citizens were expelled, Ambassadors were recalled, and as Sisko stated there wasn't a peace treaty in effect.
Just a few episodes earlier the Klingons attacked DS9 (almost a state of war). Two episodes before this the Klingons were laying mines in the area.
DS9: "Rules of Engagement":
SISKO: Advocate, how would you describe the current relationship between the Federation and the Klingon Empire?
CH'POK: There is no formal relationship between our two governments.
SISKO: What would you call us? Informal friends? Informal enemies?
CH'POK: I would say there is potential for either label, but at the moment neither is entirely accurate.
SISKO: Hmm. Interesting. Would you agree that at the moment, it is difficult for us to trust each other?
CH'POK: Difficult, but not impossible. There are things that transcend our differences. For example, we trust that this case can be decided fairly. We have faith in Admiral T'Lara's judgement.
SISKO: I'm glad to hear you have such a profound respect for the Admiral. But would it be fair to say that outside this hearing you do not entirely trust us?
CH'POK: Well, it is only prudent that we question your motives now that we are no longer allies.
SISKO: Of course. And it is only prudent of us to question your motives. After all, aren't there times when you feel it's in your best interest to deceive us?
So CH'POK gives reason for why he and the Klingons would want to bring these proceedings. But this is not a reason for the Federation to entertain this trial. Furthermore there is a request for extradition, which would require a treaty.
Khitomer Accords Article IV:
Article V: Both Parties agree that any former grievances they had with the other are now forgotten. If any individual under the rule of one Party commits an act of revenge or retribution on the other Party, that individual will be extradited to stand trial under the laws of the aggrieved Party.
Why would the Federation allow such a trial and a request for extradition under these circumstances?
STU, EU, and writers notes are welcome.