"Intelligence reports Romulans now using Klingon design."
- 2267: The Klingon Empire and the Romulan Star Empire (along with the Federation) jointly establish a colony on Nimbus III, officially declaring it "the planet of galactic peace". (Star Trek V: The Final Frontier)
- 2268: The Romulan Star Empire uses D7-class battle cruisers, which Commander Spock specifically identifies to be of Klingon design. (TOS: "The Enterprise Incident")
- 2269: Commander Spock assumes that Kor's IKS Klothos possesses effective cloaking capability. While primitive cloaking screens have been used by Klingon ships as early as 2256, cloaks effective against Starfleet sensors were thought to be an exclusively Romulan technology up until 2268. (DIS: "The Vulcan Hello"; TAS: "The Time Trap"; TOS: "The Enterprise Incident")
- 2271: Klingon Empire forces led by Kor achieve a memorable victory over the Romulans at the Battle of Klach D'kel Brakt. (DS9: "Blood Oath")
- 2285: At least one Klingon vessel definitely has cloaking capability with limited effectiveness against Starfleet sensors. As of 2287, all Klingon Birds-of-Prey are known to have a cloak. (Star Trek III: The Search for Spock, Star Trek V: The Final Frontier)
- 2292: According to Geordi La Forge, Romulans and Klingons regard each other as "blood enemies" by this year. (TNG: "Reunion")
- 2293: After the Praxis disaster, Klingon Chancellor Gorkon pursues a full peace treaty with the Federation but is assassinated by the Khitomer conspiracy, which the Romulan Ambassador to Earth, Nanclus, is a part of. Despite this attempt to prevent a Klingon-Federation alliance, Gorkon's daughter and new Chancellor Azetbur later establishes the peace treaty at the Khitomer Conference. (Star Trek VI: The Undiscovered Country)
- 2344: The Romulans attack a Klingon outpost on Narendra III. As the Federation starship USS Enterprise-C responds to a distress signal from the Klingon outpost and subsequently sacrifices itself, this attack causes the Federation-Klingon alliance to grow even stronger. (TNG: "Yesterday's Enterprise", "Redemption")
- 2346: With the help of Klingon traitor Ja'rod of the House of Duras, the Romulans commit the Khitomer Massacre, killing almost all 4,000 Klingon colonists on Khitomer. This happens at a time when, according to Worf, "the Romulans were supposed to be our [the Klingons'] allies". Over the subsequent decades, Klingon survivors were held at a Romulan prisoner camp on Carraya IV, where they adapted to a peaceful and secluded coexistence with the Romulan guards. (TNG: "The Neutral Zone", "Sins of the Father", "Birthright, Part II")
- 2350: Romulan forces fail in an attempt to board Klingon General ShiVang's flagship. That same year, later Romulan Praetor Neral's family are killed in a Klingon attack. (DS9: "Once More Unto the Breach", "Inter Arma Enim Silent Leges")
- 2367: Gowron, who is opposing the House of Duras, becomes new Chancellor. His position is quickly challenged but eventually affirmed in the brief Klingon Civil War against the Romulan-influenced, but ultimately inferior, House of Duras. (TNG: "Reunion", "Redemption")
- 2371: The renegade Duras sisters raid a Romulan outpost to acquire weapons-grade trilithium. (Star Trek Generations)
- 2372: Several outposts along the Romulan border are attacked by the Klingons. (DS9: "Hippocratic Oath")
- 2374-2375: The Klingon and Romulan Empires become part of the Federation Alliance, jointly fighting and eventually defeating the Dominion, conclusively acknowledged with the signing of the Treaty of Bajor. (DS9: "In the Pale Moonlight", "What You Leave Behind")
The existence of a "Romulan-Klingon alliance" in the 23rd century has never been explicitly stated. In "The Enterprise Incident", all that is said is that "Intelligence reports Romulans now using Klingon design." The Making of Star Trek (finished during the early part of the third season of TOS) does say, however, that the intention at the time was to have the Klingons and Romulans form an alliance against the Federation.
In developing Star Trek III: The Search for Spock, Harve Bennett originally wrote the film with the Romulans as the belligerents, whose vessel was designated the "Bird-of-Prey", but later rewrote the Romulans as Klingons, as they were determined to be more exciting. After having "Klingicized" the characters in the film, he decided to keep the vessel designation intact. Recalling his two-month binge watching of every episode of Star Trek that existed then (when TOS was the only live-action series), Bennett said, "I didn't change their ship, because I remembered a piece of trivia that stated there was a mutual assistance military pact between the Klingons and the Romulans for an exchange of a military equipment." (Starlog #103, February 1986, p. 17)
Though spotty references were made since the original reference, there were also few key indications that a loose alliance spottily existed between the two races for the next century:
- In Star Trek II: The Wrath of Khan the Neutral Zone is referenced in relation to Klingon territory while previously it was only known to relate to the divide between Federation an Romulan territories. The extension of the Neutral Zone across the Federation-Klingon border (which was an area of dispute between the two powers during TOS) would suggest Klingon-Romulan relations had evolved into something more political by no later than 2285.
- In Star Trek VI: The Undiscovered Country, Romulan delegates are one of three main groups present at the Khitomer Conference, indicating some kind of relationship between the Klingons and the Romulans. However, they are ultimately discovered, at least in part through the actions of Nanclus, to be one of the conspirators involved in the assassination of Gorkon and the attempt to kill the Federation president.
- In TNG: "The Neutral Zone", Worf says the Romulans "killed my parents in an attack on Khitomer at a time when they were supposed to be our allies." This incident occurred in 2346 and would require that the two races be both blood enemies and allied powers at the same time.
- In TNG: "Reunion", Riker says, in response to a suggestion that the Klingons and Romulans are working together, "A new Klingon alliance with the Romulans?" By saying "new", he implies that there was an "old" one at some point. The two powers were "blood enemies" for seventy-five years prior to the episode, so any former alliance would most likely have had to have ended prior to that. The Battle of Klach D'kel Brakt in 2271, mentioned in DS9: "Blood Oath", might have been at or near that end of the alliance, as it took place twenty-one years prior to them being described as "blood enemies".
According to Star Trek: The Next Generation Technical Journal, the Romulan cloaking device was acquired by the Klingons as an exchange for several D7 class battle cruisers for the Romulans during the Alliance. In fact, according to apocryphal accounts, in Seven Deadly Sins, Romulans required the power system of the D7-class ships to provide the power required to use the new improved cloaking device.