Romulan Star Empire

The Romulan Star Empire was an interstellar governmental power located in the Beta Quadrant, composed and run primarily by Romulans.

History
The Romulan Star Empire was an outgrowth of an exodus by a group of Vulcans that had rejected the Vulcan Reformation. The Vulcans that would become the Romulans left the planet in a fleet of starships, searching for a new home. Eventually, they discovered the twin worlds of Romulus and Remus, where they settled and began building a new society that would become the Romulan Star Empire.

Earth, Vulcan, and the Federation
Many centuries later, the Romulans would return to interstellar space, eventually entering into conflict with the recently warp-capable humans of Earth. This conflict flared into the Earth-Romulan War, ultimately leading to the formation of the United Federation of Planets, which included as founding members both the humans the Romulans had just fought, as well as the Romulans' own ancestors - the Vulcans. Despite the proximity of powers and the intensity of the fighting, humans and Romulans never met face to face during the four year long conflict. As a result, the link between the Vulcan and Romulan people would go undiscovered for another hundred years.

Following the conclusion of the Earth-Romulan War, the Romulans retreated into complete isolation, only reemerging in 2266 to test newly developed cloaking technology as a prelude to invasion. Though the technology was effective, the invasion was foiled and the link in Vulcan and Romulan ancestry was confirmed for the first time. (TOS: Balance of Terror)

Following the 2266 failed invasion, the Star Empire became a fitful participant in local interstellar politics, though it often used the channels of legitimate diplomacy as a cover for espionage and subterfuge. In 2293, it participated in the failed attempt to subvert the Federation-Klingon peace process that led to the signing of the Khitomer Accords. (STVI: The Undiscovered Country)

In 2311, the Star Empire precipitated the Tomed Incident, which cost over ten thousand lives and nearly resulted in another war. Following the Tomed Incident, the Star Empire retreated back into total isolation, but not before signing the Treaty of Algeron, which reinforced the existence of the Romulan Neutral Zone and prohibited the Federation from developing cloaking technology of its own. (TNG: The Neutral Zone, TNG: The Pegasus)

By 2339, there was still no official diplomatic contact between the Federation and the Star Empire. However, there had been numerous isolated incidents between Federation and Romulan forces, most of which remained classified. Following the Treaty of Algeron, the Federation also redoubled its efforts to enforce a long standing embargo against Romulan products, most notably Romulan ale, which the Star Empire continued to work to circumvent as best it could.

Klingon Empire
The Klingon Empire and Romulan Star Empire participated in a brief alliance in the 2260s which saw the exchange of Klingon ships for Romulan cloaking technology. This alliance soon collapsed, with the Romulans betraying their pact in a way viewed as unforgivably dishonorable by Klingon standards. By 2339, relations between the two powers had yet to recover, though the Klingon Empire's peace treaty with the Federation prompted a split in Klingon public opinion, with some of society believing that it would be better to ally with the Federation and others believing it better to ally with the Romulan Star Empire. This debate itself would be influenced by covert Romulan efforts designed to pull the Klingon Empire closer to its own sphere of influence and away from the Federation's.

Orion Syndicate
While not a formal interstellar government, the Orion Syndicate was largely and correctly viewed as the true seat of power within what had been known as the Orion Empire by the early 24th century. The Star Empire relied on the Syndicate heavily as a trading partner to circumvent the Federation embargo.