Federation–Klingon Cold War
Background
Rising tensions
Open war
Détente
Following the signing of the Treaty of Pahvo, a period of reconciliation between the Federation and the Klingon Empire began. While there remained mistrust and resentment between the two parties, multiple successful undertakings of peaceful, mutually-beneficial collaboration marked a dramatic improvement in their diplomatic relationship. One significant breakthrough was the Federation–Klingon Free-Trade Agreement of 2256, a treaty negotiated by Dak'Rah, son of Ra'Ul, Federation Ambassador to the Klingon Empire.
By the mid-2260s, however, tensions were once again on the rise. Federation–Klingon détente appeared to be fast dissolving. Things came to a head in 2267, when arduous negotiations between the Federation and the Klingon Empire broke down, leading the Klingons to launch a series of small surprise attacks on numerous Starfleet vessels along the Federation–Klingon border and quickly seize the strategically located neutral planet of Organia.
However, the Klingons' plans were foiled by a third-party intervention. All military forces of both the Klingon Imperial Fleet and Starfleet were immobilized by the Organians, who revealed themselves to be immensely powerful, incorporeal beings. The Organians oversaw treaty negotiations between the Federation and the Klingon Empire, putting an end to the short-lived war. While the new treaty ended the fighting, it did little to ease the tensions between the two adversaries, prompting the Klingons to seek other avenues to undermine the Federation.
Klingon–Romulan alliance
Shortly following the Organian incident, the Klingon Empire negotiated an anti-Federation military alliance with the usually-isolationist Romulan Star Empire. The alliance involved strategic coordination, intelligence-sharing, and a technology exchange. The Klingons acquired numerous Romulan Birds-of-Prey equipped with primitive cloaking devices. In return, the Romulans acquired numerous Klingon D7-class cruisers equipped with class 4 warp drives.
In 2282, the Klingons discovered that the Federation had developed the Genesis Device, a terraforming technology that could also be used to eradicate life. In his search for the secret of Genesis, the Klingon Commander Kruge was killed by Starfleet Admiral James Kirk. Despite Kirk's rogue status at the time, the Klingon Ambassador to the Federation proclaimed, There shall be no peace as long as Kirk lives!
When the Federation refused to extradite Kirk, the Klingon Empire withdrew from all existing agreements with the Federation, including the Treaty of Pahvo and the Treaty of Organia. The Federation was braced for a new war with the Klingon Empire.
However, by this time, the Klingon–Romulan alliance was beginning to break down. The Romulans' new Klingon warp drive technology—in addition to more-than-doubling the speeds of their warbirds—generated enough energy to power their newest, most advanced cloaking system. Having exhausted the benefits of the alliance and eager to avoid a bloody war with the Federation, the Romulan Star Empire backed out of their treaty with the Klingons and established diplomatic relations with the Federation. With the end of the Klingon–Romulan alliance, the Klingon Empire, now more cut off from the rest of the galaxy than ever before, entered into a period of total isolationism.