- 2267: The Romulan Star Empire uses Klingon-designed D7-class battle cruisers. That same year, 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'. (TOS: "The Enterprise Incident", Star Trek V: The Final Frontier)
- 2271: The Empire achieves a memorable victory over the Romulans at the Battle of Klach D'Kel Brakt. (DS9: "The Sword of Kahless")
- 2292: By this year, the Romulans are considered "blood enemies" in the Klingon Empire. (TNG: "Reunion")
- 2293: After the Praxis disaster, Klingon Chancellor Gorkon pursues a full peace treaty with the Federation but is assassinated by the Romulan-influenced Khitomer conspiracy. Despite this attempt to prevent a Klingon-Federation alliance, Gorkon's daughter and new Chancellor, Azetbur, later concludes the peace 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 the Klingon distress signal and subsequently sacrifices itself, this attack caused the Federation-Klingon alliance to grow even stronger. (TNG: "Yesterday's Enterprise", "Redemption")
- 2346: With the help of the Klingon traitor Ja'rod of the House of Duras, the Romulans commit the Khitomer Massacre, killing almost all of the 4,000 Klingon colonists on Khitomer. This happened at a time when, according to Worf, "the Romulans were supposed to be our [the Klingon's] allies". (TNG: "The Neutral Zone", "Sins of the Father")
- 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")
- 2374-2375: The Klingon and Romulan Empires become part of the Federation Alliance, jointly fighting and eventually defeating the Dominion with the Treaty of Bajor.
The existence of a "Romulan-Klingon alliance" in the 23rd century has never been explicitly stated. In "The Enterprise Incident", all that was said was 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. The only real indication in any series that an alliance was ever in effect is in "Reunion", where Riker says in response to a suggestion that the Klingons and Romulans were working together: "A new Klingon alliance with the Romulans?" By saying "new" it implies that there had been an "old" one at some point. The two powers were "blood enemies" for 75 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 the Star Trek: Deep Space Nine episode "Blood Oath", might have been at or near that end of the alliance, as it took place 96 years prior to the episode. However, since another 21 years would pass prior to being blood enemies, it is likely the alliance attempted to remain intact thereafter.
It is possible that the Romulans and Klingons had a second alliance as well. In "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." That was 2346, and would require that the two races be both blood enemies and allied powers at the same time. This is possible as governments will often put aside philosophical or historical differences in favor of political expediency or gain, but it is highly speculative. However, in Star Trek VI: The Undiscovered Country, there are Romulan delegates present at the Khitomer Conference, so it is possible that some kind of relationship between the Klingons and the Romulans also extended from those events (even though Nanclus turned out to be one of the conspirators involved in the assassination of Gorkon and the Federation president).
According to the 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.