First Federation-Klingon War
The First Federation-Klingon War was a conflict fought between the United Federation of Planets and the Klingon Empire in 2267. It was the result of years of tense build-up in a cold war situation, but was ultimately inconclusive because the fighting was abruptly ended by the Organians.
Shortly before stardate 3198.4, ongoing negotiations between the two sides were in danger of breaking down, and open warfare was becoming an unwelcome likelihood. Starfleet Command expected that the Klingons would launch a surprise attack after they issued an ultimatum demanding that the Federation withdraw from all disputed regions along their mutual border, including Donatu V, Sherman's Planet, Organia, the Archanis Sector, and other territories. When the Federation refused the demands, the Klingons launched an immediate attack – even before an official declaration of war was issued.
The Klingon invasion of Organia was the major campaign during the conflict, cumulating in the Organians forcing an end to the hostilities. They then addressed both the Federation Council and Klingon High Council and announced the unilateral imposition of the Treaty of Organia. The treaty established guidelines for peaceful interaction between the two powers and the resolution of territory disputes.
(References: The Original Series: "Errand of Mercy", "The Trouble With Tribbles")