The Khitomer Conference was a diplomatic conference held in 2293 by the major Alpha Quadrant powers - namely the United Federation of Planets, the Romulan Star Empire, and the Klingon Empire on the planet Khitomer - a Klingon held world near the Klingon-Romulan border. This conference would be one of the most important conferences of the late 23rd century, and represented a major shift in the political direction of the quadrant.
Captain James T. Kirk and the crew of the USS Enterprise (NCC-1701-A), as well as Captain Hikaru Sulu and the crew of the USS Excelsior (NCC-2000) were instrumental in preventing the assassination of Federation President Ra-ghoratreii at this conference. They also exposed a conspiracy between certain officials in the Klingon, Romulan, and Federation governments who did not want peace between the major powers. Captain Kirk was also finally able to grant forgiveness to the Klingons for the death of his son, David Marcus - stating that his son's faith had finally been restored.
During the conference, Captain Spock met the Romulan delegate Pardek, who had been assisting Ambassador Nanclus until Nanclus had been exposed as a member of the conspiracy and arrested. (TOS novelization of Star Trek VI: The Undiscovered Country) Seeing an opportunity to manipulate Spock for political advantage, Pardek cultivated a friendship with Spock. Decades later, Pardek used this friendship with Spock to expose Romulan dissidents - many of whom were then killed by Praetor Neral's elite soldiers. (TNG novelization of Unification)
Another delegate present at the conference was Curzon Dax, who helped in negotiating the agreements between the two powers. These agreements, which would become known as the Khitomer Accords would become one of the foundations of the Klingon-Federation Alliance. They would remain in effect until Chancellor Gowron withdrew from the Accords following the Federation Council's refusal to support the Klingon invasion of Cardassia. (DS9 episode: "The Way of the Warrior" and You Are Cordially Invited...)