Federation-Klingon War (2405-2410)

The third  began in early 2405 following a period of worsening relations between the United Federation of Planets and the Klingon Empire around the turn of the 25th Century.

Brush wars

 * 2383:
 * Relations between the two powers begin to degrade when the Federation considers formally censuring the Klingon Empire for its attack on Khitomer the previous year. Though the motion is defeated, the Klingons take offense to the very existence of a vote and recall their ambassador. Chancellor Martok does, however, choose not to eject Federation ambassadors from Klingon space despite demands from hardliners on the High Council.
 * 2385:
 * All non-Klingon residents of Qo'noS are expelled as a "safety measure."
 * 2387:J'mpok.png]]
 * Relations between the two powers degrade further following the destruction of a Klingon battle group led by Worf during an attack on the Narada. Hardliners on the High Council, led by Councilor J'mpok, believe Martok's decision to give a former Federation official command of the mission rather than a general of the Klingon Defense Force may have been a trick to weaken the Empire. Martok denies this and refuses to retaliate against the Federation.
 * 2389:
 * Klingon attacks on Romulan and Gorn systems and ships become more frequent. Starfleet assigns ships to protect the Klingon-Romulan border, resulting in several isolated skirmishes between Federation and Klingon ships.
 * 2390:
 * The Federation offers to mediate peace talks between the Gorn and the Klingons at a neutral location. Martok responds that the Klingons will not participate in any negotiations until Starfleet withdraws all of its ships from the Romulan-Klingon border. The Federation Council eventually agrees.
 * 2391:
 * Representatives from the Klingons, Gorn and Nausicaans converge on Deep Space K-7 for the conference, and the Federation sends some of its most celebrated diplomats to oversee the talks. Unfortunately, the peace talks end following the detonation of a bomb that seriously injures Ambassador Zogozin of the Gorn.
 * 2392:
 * Another peace conference is held, but it also fails when the Gorn begin an invasion of Ogat, a Klingon colony.
 * 2393:
 * Martok is deposed by J'mpok, who kills him and declares himself the new Chancellor. One of his first acts is to bar Worf from the Great Hall. The regime change is a major cause for concern for the Federation, and President Aennik Okeg's invitation to meet with J'mpok is declined.
 * 2394:
 * The Federation condemns the Klingons' alliance with the Orion Syndicate as support for criminals.
 * 2395:
 * Following the destruction of the USS Kelso, and the discovery that it had been testing an experimental cloaking device, J'mpok recalls the Klingon ambassador to the Federation back to Qo'noS for "strategic discussions."

The Undine conspiracy

 * 2395:
 * It is discovered by Ja'rod that the Undine are actively infiltrating every major political entity of the Alpha and Beta quadrants.
 * 2396:
 * Captain T'Vix of the is removed from duty along with her first officer and chief of security. Though the reasons for this are kept hidden by Starfleet Intelligence, it is eventually revealed that they had been replaced by Undine infiltrators.
 * 2398:
 * The Klingon High Council refuses to allow its people to apply for dual citizenship with the Federation.
 * President Okeg reaches out to the Klingons during his re-inauguration speech.
 * The third Klingon-Gorn peace summit is interrupted by another bombing which kills Klingon ambassador K'mtok. The Klingon and Gorn governments refuse to participate in any more peace talks.
 * 2399:
 * The Klingon Empire begins a massive invasion of Gorn space after Ja'rod - now captain of the - provides the High Council with evidence that the Hegemony's government is under direct control of the Undine. J'mpok sends a message to President Okeg, asking for Federation assistance as spelled out in the Khitomer Accords. Instead, the Federation Council condemns the invasion and demands that the Klingons withdraw from Gorn space and return to the negotiating table. J'mpok pulls the Empire out of the Khitomer Accords and recalls all Klingon ambassadors back to Qo'noS, beginning a true cold war between the two powers.
 * 2400:
 * Attempts to restart the Federation-Klingon alliance are unsuccessful, and J'mpok repeatedly refuses to meet with President Okeg.
 * 2401:
 * Lethean mercenaries expose Klingon Councilor Konjah as an Undine infiltrator. The spy is killed, and Konjah's house disbanded.
 * Tensions continue to mount between the Federation and the Klingons. A few instances of Starfleet officers taking matters into their own hands are reported, such as Captain Zachary MacAllister's attempt to help the Gorn fight the Klingon advance because he believed the Klingons would come after Starfleet next.
 * A number of current and former Starfleet officers including Ambassador Worf, Admirals, and  and retired Captains  and Tuvok appeal to the Federation Council to reconsider its position on the Klingon-Gorn War. When they fail, Worf resigns his position with the Federation in protest and returns to Qo'noS.
 * 2402:
 * The Klingons breach the Gorn lines and blockade their homeworld, Gornar.
 * 2403:
 * The network of transport inhibitors protecting Gornar finally fails, allowing the Klingons to conquer and occupy the planet.
 * After arriving on Gornar, J'mpok orders a global broadcast during which the Klingons reveal that the Gorn prime minister, head of military intelligence and multiple other high-ranking government officials are Undine infiltrators. The infiltrators are then immediately executed.
 * J'mpok then offers the Hegemony self-rule as long as it swears fealty to the Klingon Empire. King Slathis agrees, and receives a non-voting seat on the High Council.

2404-2408

 * 2404:
 * J'mpok announces that he is invoking ancient claims to the Hromi Cluster and the sectors surrounding it, an area that had been in Federation hands for decades. He orders all Federation citizens to evacuate within three months.
 * 2405:
 * President Okeg refuses to obey J'mpok's ultimatum, and the Klingons invade the Korvat System, marking the beginning of the fourth.
 * The Gorn join the war as well, attacking Sherman's Planet and Deep Space K-7.
 * Many Nausicaan tribes sign a non-aggression pact with the Empire.
 * 2406:
 * Klingon fleets invade the Archanis Sector. Both forces are evenly matched, resulting in a seige that lasts well into 2407.
 * J'mpok repeatedly refuses to consider a peace summit.
 * 2408:
 * Klingon and Gorn forces invade Cestus III and surrounding colonies, and Starfleet masses a large fleet to repel them.

2409-2410 (in-game)
''Much of Star Trek Online takes place during the war. As a result, expect spoilers ahead.''
 * 2409:
 * The Gorn become obsessed with reclaiming former colony worlds, attacking Federation-held systems such as Tostig and Kassae.
 * Both the Federation and the Klingon Empire agree to recognize the Romulan Republic as a legitimate interstellar power, and it is hoped their alliances with the Republic will help institute an armistice.
 * B'vat.pngon Admiral [[B'vat]] becomes convinced that the conflict is the only thing holding the Klingon Empire together, and plots to extend it indefinitely by developing devastating new weapons and technology to use against Federation civilian populations. He also attempts to reactivate a derelict Planet Killer, though the weapon is eventually destroyed by Starfleet.
 * Klingon fleets conduct daring raids on Utopia Planitia Shipyards and Starbase 24.
 * B'vat travels back in time with a kidnapped Miral Paris to 2260 where he attempts to change history in the Empire's favor. He is defeated and killed by the player, who had followed him back in time, and Miral Paris rescued.
 * Following the Borg Collective's advance in the Gamma Orionis Sector Block, growing evidence of an Iconian conspiracy, the return of a lost Dominion fleet from 2374, and the discovery of the Solanae Dyson Sphere in the Delta Quadrant, Starfleet and the Klingon Defense Force sign a limited cease-fire and begin conducting joint operations such as Task Force Omega and Dyson Joint Command.
 * 2410:
 * Devastating Undine attacks on the Sol System and Qo'noS lead to the two sides cooperating to protect their homeworlds. Va'Kel Shon's willingness to sacrifice himself in defense of Qo'noS in particular led to Quinn and J'mpok agreeing to cease hostilities; a decision solidified by an Iconian wiping out much of the Klingon High Council.
 * The two sides eventually formed the Alliance during their exploration of the Delta Quadrant, inviting other species to join.