Their home world is Romulus, and, with several other systems, their political organization is called the Romulan Star Empire. The Romulan Empire is one of the largest planetary nations in the Beta Quadrant of our galaxy. Planets known to be under Romulan jurisdiction include Romulus, Remus, Regulus and Nimbus III. Very little is known about the Romulan nation's extent, partially because the empire has usually been at war with the United Federation of Planets. The Empire and the Federation have been separated from each other by the Romulan Neutral Zone for many years.
The Romulan species is actually a splinter group of Vulcans who refused to accept the Vulcan philosopher Surak's teachings of completely suppressing one's emotions. Around 400 CE they split off from Vulcan society and established colonies in the Romulus system with home planets Romulus and Remus.
In the mid 1900s the Romulans and Vulcans entered into war which lasted until 2044. Later in 2150 the humans would make first contact with the Romulans and four years later the Earth-Romulan war began.
In 2160 the Romulans and the humans sign a treaty establishing a neutral zone one light-year wide between their territories and in 2161 the humans formed the United Federation of Planets.
About a century later the Romulans invented a new piece of technology called a cloaking device which made their ships invisible to sensors. They then proceeded to break the treaty by attacking several Federation outposts. In response to these attacks, the Starship Enterprise tracked down the ship carrying out the attacks and destroyed it.
Several years later, in 2267, the Romulans entered a treaty with the Klingon Empire, and received several D7 battlecruisers, which the Romulans upgraded into extremely deadly war machines, in exchange for cloaking technology.
In 2293 the Klingon moon Praxis exploded and the Klingons entered into a treaty with the Federation. This infuriated the Romulans who, in 2311, attacked the Federation. Following the attacks the Romulans and the Federation created a new treaty, the Treaty of Algeron, which reestablished the neutral zone and prohibited the Federation from developing cloaking technology and there were no further incidents for fifty years.
In 2344 four newly invented Romulan Warbirds attacked the Klingon outpost Narendra III. The USS Enterprise-C responded to the distress call from the outpost and engaged the ships. However good the Enterprise C's intentions the starship was defeated but not destroyed, and the ship along with its survivors (among them Tasha Yar) were taken back to Romulan territory. Rumours circulated in the Federation that the Enterprise's broken hull was displayed on Romulus, to enlighten and boost moral of its Romulan Academy students. No doubt the presumed destruction of the Enterprise C was a saddening element for the Federation. The Klingons thought this action was honorable and thus this strengthened relations between the Klingon Empire and the Federation but also initiated a war between the Romulans and the Klingons, which lasted several years. This action infuriated the Romulans, who in turn launched a raid on Qo'nos and destroyed 5 K'vort class birds of prey, 10 Vorch'a Battle Cruisers and 7 B'rel class birds of prey while only losing 5 Warbirds.
In 2366 the Romulans attempted to trick the USS Enterprise-D into crossing the neutral zone where they had two Romulan Warbirds waiting to destroy the Enterprise. Captain Picard, however, was assisted by two Klingon Birds of Prey; the Romulans, evenly matched, decided that the price of the Enterprise's destruction was too high and therefore retreated. ("The Defector")
In 2367, the Romulans attempted to brainwash Federation lieutenant commander Geordi LaForge but the plan was exposed and failed. ("The Mind's Eye")
Later in 2367 the Klingon Civil War began and the Romulans secretly backed the house of Duras, but Captain Picard of the Federation discovered this and convinced Federation-friendly Gowron to attack the Duras forces hard in hopes that they would request the Romulans bring in reinforcements, which would be detected by a Federation blockade, thus revealing to everyone that Duras was backed by the Romulans. The plan worked and Duras quickly lost all support for the Chancellor's seat. ("Redemption, Part I")
Sometime during or before 2368, Ambassador Spock, a Vulcan, was on Romulus, working with an underground movement for the reunification of the Romulans and Vulcans. The Romulans, however, saw this as a chance to conquer the Vulcans and sent a fleet of ships to Vulcan, but their intent was discovered and the fleet destroyed. ("Unification")
In 2371 the Romulans allowed the Federation to borrow a cloaking device under the condition that they only used it in the Gamma Quadrant, where a new threat is emerging with Jem'Hadar and the Dominion. ("The Search")
The Romulan intelligence agency, the Tal Shiar, allied with the Cardassian equivalent, the Obsidian Order, in late 2371 and launched a preemptive strike on the Dominion, but the Dominion destroyed them both and it was revealed that Colonel Lovok, the Tal Shiar officer leading the attack was in fact a Dominion agent ("The Die Is Cast"). The entire fleet of 20 Warbirds and 20 Kelvan class ships were destroyed, while the Dominion lost only 46 attack cruisers.
Soon after, the Klingons went on a rampage and attacked the Cardassians and several Romulan outposts. The Romulans sent a warbird to join the Klingon and Federation front guarding the Bajoran wormhole but when a Founder disguised as Julian Bashir made a failed attempt to destroy Deep Space Nine, the Romulans signed a nonaggression treaty with the Dominion and went home.
In 2374 Captain Sisko and Garak, in an attempt to trick the Romulans into joining the war against the Dominion, showed Romulan Senator Vreenak fake recordings of a Dominion strategy meeting which revealed that the Dominion intended to attack the Romulans after taking the rest of the Alpha Quadrant. Vreenak examined the recordings and, discovering the deception, leaves the station to inform the Romulan government. Garak, however, sabotages Vreenak's ship, killing Vreenak. Later, when the Romulans recover the recording from the wreckage the defects that allowed Vreenak to discover their fraudulence are assumed to have been caused by the explosion, which is blamed on the Dominion. This, of course, is all top secret and only two-three people are aware that the tapes aren't real. ("In the Pale Moonlight")
Now that the Romulans have joined the Klingon-Federation alliance, they establish a presence on Deep Space Nine in 2375 and attempt to stockpile weapons on a Bajoran moon. However, Colonel Kira stops them with a Bajoran fleet, with the help of Federation Admiral Ross's statement that the Federation will back the Bajorans in this matter. ("Shadows and Symbols")
In 2375 Section 31 attempts to assassinate the Romulan Koval, the new head of the Tal Shiar. The plan is stopped when Bashir, who is being used by Section 31 as a spy, asks Senator Cretak for assistance in stopping the plot. ("Inter Arma Enim Silent Leges")
In 2375 the Federation-Romulan-Klingon fleet heads to Cardassia Prime to finish off the Dominion's presence in the Alpha Quadrant. The fight isn't going well until the Cardassians fleet turns and joins the fight against the Dominion. This leads to the defeat of the Dominion and an end to the Dominion War. ("What You Leave Behind")
In 2379 the Romulan Senate was briefly overthrown in a Reman coup led by Shinzon. The Praetor and the senate were killed by thalaron radiation which eminated from a handheld device. Shinzon became Praetor and the Remans took over the Romulan Senate but were overthrown by Captain Jean-Luc Picard and the Romulans regained control over the government. Relations between the Romulan Star Empire and the United Federation of Planets have improved because of the assistance rendered by Picard. ("Star Trek: Nemesis")
The author Diane Duane wrote a number of novels about the genesis and culture of the Romulan people, chief of which are My Enemy, My Ally, and The Romulan Way. Duane also developed a Romulan language in these novels, but which has not been used on the Star Trek series. Otherwise, not much is known about the Romulans. Data implies in "Nemesis" that the Romulans use a caste system.
See also: Star Trek