The Klingon-Romulan Alliance was an alliance between the Klingon Empire and the Romulan Star Empire formed in the mid-23rd century. The alliance offered both sides a strategic advantage in their conflicts with the United Federation of Planets, and resulted in a technology exchange between the two powers. (TOS comics: "The Hollow Crown, Issue 1", "The Hollow Crown, Issue 2")
The formation of the Alliance has had two distinct tellings, one in which Vrax was an old, wizened Praetor and another in which a new Praetor rose to power, so the formation of the alliance has been split into two sections telling each of the stories.
In 2267, the Romulans were developing new cloaking devices but they required far more power than older versions and the Romulans didn't have ships that could produce that much power, so Proconsul Toqel suggested an alliance with the Klingons as they had ships that could produce sufficient power. Her plan was endorsed by Praetor Vrax and given the go ahead by the Senate, so she began negotiations with Grodak, a minor Klingon diplomat. These negotiations would lead to a full alliance, in which much technology was shared and the two empires would test a new cloaking device fitted warship, the IKS Vo'qha, against the Federation's USS Defiant. (ST novella: The First Peer)
New Praetor continuity
The first seeds of the alliance were sown in 2266 when the Klingons became aware the Romulans were developing a new cloaking device and sent Korlat to manipulate the recently ascended Praetor into forming an alliance; hoping to acquire the strategically advantageous new technology for themselves. Korlat also used his relationship with the Praetor to goad him; feeding the young ruler's lust for power and manipulating him into an antagonistic relationship with the Federation; inspiring the Praetor to launch his new "ghost ship", the ChR Gal Gath'thong, to attack Federation targets along the Romulan Neutral Zone. (TOS comic: "Alien Spotlight: Romulans")
Following the formation of the Organian Peace Treaty the Klingons had further motivation to strengthen the new alliance; hoping to use the Romulans to indirectly make war against the Federation; something they could no longer do themselves directly as a consequence of the treaty. (TOS comic: "The Hollow Crown, Issue 2")
Following the loss of the Romulan ghost ship in battle with the USS Enterprise during in its attacks along the Neutral Zone the Klingons, in a plan spearheaded by Koloth, saw a new opportunity to strengthen their hold over the Romulans, and began to pull together elements in the Romulan political scene that would usher in a new Praetor even more willing to be influenced by them.
Seeing the deceased ghost ship commander, Keras', family's lust for revenge Koloth suggested to the Praetor that he should pursue the widowed Lady Arenn; to make her his queen, and in doing so nullify her political opposition to him. The Praetor was easily manipulated and a year later the two were married. Of course the Klingons were well aware that the politically astute widow who had lost her husband to the Praetor’s war lust was not so easily tamed, and anticipated her revenge would be the end of the praetor - correctly so. She poisoned him on their wedding night before destroying herself and the Imperial Palace, with an explosive device of Klingon origin; hoping to sour the alliance. (TOS comics: "The Hollow Crown, Issue 1", "The Hollow Crown, Issue 2")
Meanwhile however the Klingon Kloor had been twisting Keras' son, Gaius; providing him with falsified communications logs from the Battle of Icarus IV which painted James T. Kirk as a blood thirsty villain, willing to kill Keras even when the Romulan had surrendered. This further provoked the already enraged young Romulan. The Klingons also made sure the Praetor announced Gaius as his heir as soon as Arenn and the Praetor had been married, thus securing their ideal new praetor in power, ready and willing to strike out for revenge against the Federation. In their final act to secure their power over Gaius the Klingons falsified mind-sifter readings to implicate Arenn's co-conspirators so that it appeared they had conspired to murder Arenn and the Praetor against her will, to try and destabilise the alliance. (TOS comic: "The Hollow Crown, Issue 2")
After installing Gaius as Praetor the Klingons' plan seemed to be working, as the Romulans went to war with the Federation and the Klingons were able to participate in the war effort unhindered by the Organians by operating unmarked ships that could pass as a Romulan D7-class vessels. (TOS comic: "Schism, Issue 1")
Hoping to track down James Kirk and take his revenge, Gaius had a huge new starship built, from where he planned to command his Empire's invasion into Federation space. However, as the Romulans stepped up their attacks the Federation also altered its strategies; using a cloaking device stolen from a Romulan vessel in 2268 the USS Yorktown began patrolling the Neutral Zone. Along the Neutral Zone the Yorktown was confronted by Romulan and Klingon vessels, as well as the Praetor’s new ship, the ChR Keras. A disagreement over which side should claim the kill of the Yorktown led to the Klingons and Romulans turning on each other, and the battle then running three ways as further Federation vessels entered the fray. This chaotic battle finally provoked the Organians to intervene and reassert the Organian Peace Treaty, extending it to cover the Romulans and stop the Klingon manipulations. (TOS comics: "Schism, Issue 2", "Schism, Issue 3")
When the Organians revealed how the Klingons had manipulated him, Gaius went mad, and upon his return to Romulus his pregnant wife, Elee'n, who had already been sceptical of the alliance, took control of the Empire and ordered the Klingons to leave; considered the alliance at an end. (TOS comic: "Schism, Issue 3")
Despite this, and engagements between Klingon and Romulan forces, such as the Battle of Klach D'Kel Bracht in 2271, the Alliance was still in place in the late 2270s. (DS9 episode: "Blood Oath"; TOS - Star Trek II Short Stories short story: "Under Twin Moons")
Effects of the alliance
One of the key benefits of the alliance to both sides was a technology exchange. As desired the Klingons were granted access to Romulan cloaking devices, with the space forces of both sides working together to install the devices on Klingon vessels, and in war games to test their effectiveness and improve tactics. In addition to cloaking devices the Klingons also received plans for Romulan designed plasma torpedos. (TOS comic: "The Hollow Crown, Issue 1", ST novella: The First Peer)
From the Klingons the Romulan Imperial Fleet received new D7-class starship, dubbed K-class by the Romulans. The Romulans were also given an assortment of Klingon technology; weapons, transporters, and the mind-sifter. (TOS comics: "The Hollow Crown, Issue 2", "Schism, Issue 3")
- The ST reference: Starship Spotter describes how the B'rel-class Klingon bird-of-prey, first in service in 2270, was based on a Romulan design. The Klingons presumably gained access to the Romulan design thanks to the alliance, but this is not stated by the source.
As noted by Praetor Gaius' wife Elee'n, while the Klingons showered the Romulans with technological benefits they were quick to turn their noses up at Romulan culture: The Klingons were known to openly mock Romulan tradition, amuse Romulan women, and even break Romulan law - returning to their ships to avoid the consequences. Elee'n saw the Klingons less as allies and more as an occupying force. (TOS comic: "Schism, Issue 1")
In an alternate timeline in which Doctor Noonien Soong's dream of widespread acceptance of artificial life in the form of Soong-type androids was realised, the Klingon-Romulan Alliance was reformed when the House of Duras and its supporters won the Klingon Civil War. This combined power worked against the United Federation of Planets. (TNG - Myriad Universes novel: Brave New World)