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As of 2268, the Romulan Star Empire used D7-class battle cruisers of Klingon design and equipped them with cloaking technology

Klingon General Korrd with Human and Romulan ambassadors Talbot and Dar aboard the Enterprise in 2287

B'Etor and General Movar collaborating during the Klingon Civil War in 2367

A Romulan-Klingon couple in the prison camp on Carraya IV in 2369

Romulans, Klingons and Starfleet personnel bickering and discussing their strategy against the Dominion in 2374

A joint Romulan-Klingon-Federation fleet departs Deep Space 9 to invade Cardassia in 2375

"That's a Klingon ship! But it couldn't be, not in this area."
"Intelligence reports Romulans now using Klingon design.

Romulan-Klingon relations shifted between being "blood enemies" and titular "allies" in the 23rd and 24th centuries.


Background information

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.

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