Klingon-Romulan Alliance

The Klingon-Romulan Alliance originated in 2266, after the prototype T'liss-class Bird-of Prey of the Tal Praenax was destroyed in battle by the USS Enterprise. (TOS: "Balance of Terror")

Out of concern for the future of the Romulan Star Empire, Praetor Colius opened a consensus with the Romulan Senate in opening a dialogue with friendly houses within the Klingon Empire, whom shared a mutual distrust with the United Federation of Planets as they engaged in territorial expansion. A year later, a diplomatic opportunity arose as the Klingon Empire was forced into signing the Treaty of Organia. (TOS: Errand of Mercy)

Shortly after the ratification of the Treaty of Organia, the Federation proposed a diplomatic station on Nimbus III, where the Federation, the Klingon Empire, and the Romulan Empire to mutually develop the planet, in the hopes that it would serve as a model for a new age of peace and understanding. But on the day of founding of the ambassadorial station, unbeknownst to the Federation Ambassadors, dignitaries of the Romulan Empire and the Klingon Empire met in secret and signed a formal declaration of allegiance.

Tasked with this negotiation was Kor, representing the Klingon Empire, and Hectavious, representing the Romulan Star Empire. It was agreed upon that in turn for Romulan cloaking technology and certain intelligence information, the Klingon Empire would give the Romulans twelve D7-class battle cruisers as well as technical schematics for ground weaponry. (Star Trek Online Foundry: The Battle of Klach D'kel Brakt)

The End of the Alliance
In 2271, the Klingon-Romulan Alliance was dissolved when a Romulan plot was discovered on the Son'a planet of Tsi's'du, where they were using the Alliance as a means to quickly modernize the Romulan Navy without the need of years of espionage and research. They built a secret staging point on the Son'a mining world and were developing a bio weapon derived from the Klingon Augmentation Virus, which was intended to obliterate the Klingon species and allowing a quick annexation of Klingon territory through the guise of sending medical aid.

In retaliation, the Klingon High Council gave Kor command of the Klingon fleet, consisting mainly of several new models of the D7-class battle cruisers and won a decisive victory, known as the Battle of Klach D'kel Brakt.