First Khitomer Accords
The explosion of the Klingon moon Praxis in 2293 caused severe ecological disasters on the Klingon homeworld and an almost complete shutdown of the Klingon economy. Following these events, the Klingon chancellor Gorkon initiated peace talks with the Federation in which Captain Spock served as a special envoy to the Empire. The peace conference was first scheduled to be held on Earth, but was later relocated due to Chancellor Gorkon's assassination on his way to Earth. The new venue was Camp Khitomer, on the Klingon planet Khitomer, which was kept secret in order to prevent further assaults. Despite further actions by a Klingon/Federation/Romulan conspiracy, the Khitomer Conference was a success and a turning point in Klingon-Federation relations. (Star Trek VI: The Undiscovered Country)
In 2344, the Battle of Narendra III had a decisive influence on interstellar history. As the Klingons were betrayed by the Romulan Star Empire and were deeply impressed by the courage of the Enterprise-C, this event had the effect of finally ending Romulan-Klingon relations and solidifying Federation-Klingon relations. (TNG: "Yesterday's Enterprise")
The following year, Gowron reinstated the treaty at Captain Benjamin Sisko's request when an alliance between the Dominion and the Cardassian Union threatened to throw the Alpha Quadrant into war. (DS9: "By Inferno's Light")
Second Khitomer Accords
It is possible that the second accords may be the 'peace treaty' referred to by Richard Castillo in "Yesterday's Enterprise", suggesting a signing date in the 2340s. The second accord could be the formal alliance between the Federation and the empire referenced in The Next Generation. It is also possible that it occurred after the events of DS9: "By Inferno's Light" in which the Federation and the Klingon Empire renewed their alliance. This is supported by the fact that Chancellor Gowron and Admiral Ross said in DS9: "When It Rains..." they had met at Khitomer.
The Khitomer Accords were expanded to include the Imperial Romulan State and the Ferengi Alliance in the novel A Singular Destiny, set in 2381. The Cardassian Union and Talarians were also invited to join.
In the year 2399, the Khitomer Accords break down between the Federation and the Klingon Empire, and all out war is imminent, according to the storyline in Star Trek Online. Khitomer Vortex is the name of a Special Task Force mission in the game available only to flag officer-ranked players. In this mission, the player must join with 4 other players to prevent the Borg from utilizing a temporal vortex and thus corrupting the timeline.