(covers information from several alternate timelines)
The Time of Awakening
Vulcans were once an extremely violent and emotional people, waging almost constant warfare on one another. (TOS: "Balance of Terror"; ENT: "The Forge") It is believed that, at one point, 90% of the planet Vulcan was at war. As their level of technology improved, the Vulcans eventually reached a point where their violent nature threatened the species' extinction. Terrible weapons were created, like the Stone of Gol, a psionic resonator which is believed to have been destroyed by the gods on Vulcan during the Time of Awakening. It was actually disassembled and its parts scattered, some of which wound up on other worlds. (ENT: "Awakening"; TNG: "Gambit, Part II")
Finally, around 370 AD, the ancient philosopher Surak, revered as the father of Vulcan civilization, led his people in a great reformation to reject their emotions in favor of a philosophy that embraced pure logic. He demonstrated enormous courage, working for peace in the face of war. (TOS: "The Savage Curtain")
One group, known as "those who march beneath the Raptor's wings" (the future Romulans), did not accept Surak's teachings and split off from the Vulcans at this time. (TNG: "Gambit, Part II"; ENT: "Awakening")
Formation of an empire
Around 470 AD, the dissidents had left Vulcan. Upon their travels, a second group splintered from the main group of dissidents and became known as the Debrune. Much information about this group was later lost, as they apparently no longer existed by the 24th century. (TNG: "Gambit, Part I")
The original dissidents named their race the Romulans and eventually settled on the twin planets later named Romulus and Remus. Romulans were considered a group of thugs at the time and warp drive was regarded as the key technology that allowed the founding of the aggressive Romulan Star Empire. (TNG: "Unification I"; Star Trek: Insurrection)
At some point after the formation of the Empire, the Romulans once again became embroiled in conflict with their Vulcan cousins. It was unknown at the time, but Q later claimed that one of the "self-destructive stunts" of a philosopher Q, also known as Quinn, is what instigated this conflict. (VOY: "Death Wish")
The Empire's first recorded contact with Humans occurred in 2152. The Starfleet vessel Enterprise NX-01, discovered a planet recently annexed by the Romulans, and was damaged by a cloaked spatial mine, part of a minefield surrounding the planet. A Bird-of-Prey warned the Enterprise off without a further exchange of fire. (ENT: "Minefield")
Romulans found advantage in the fractious relationships of Vulcan, Andoria, and Tellar existing before 2154. Earth, rising as a regional power in the early 2150s, practiced a mediating, even unifying, influence on its neighbors. The prospect of an alliance contrary to Romulan interests led to their covert attempts to destabilize the region and the fragile peace. (ENT: "Babel One", "United")
A collaboration with V'Las of the Vulcan High Command was part of a decades-long effort to "reunify" the cousins, presumably under Romulan rule. However, a group of Vulcans named Syrrannites followed the logical teachings of Surak and were a threat to the Romulan plans of reunification. Secretly working with a Romulan deep-cover agent within the Vulcan High Command known as Major Talok, V'Las framed the Syrranites for the destruction of the United Earth Embassy on Vulcan. The conspiracy ultimately led to the nearly-disastrous Battle of Andoria between Vulcan and the Andorian Empire in 2154. (ENT: "The Forge", "Kir'Shara")
After V'Las was deposed, Romulans precipitated the "Babel Crisis" by deploying telepresence-operated drone-ships equipped with chameleon-like camouflage systems. Though briefly wreaking havoc on regional shipping and diplomacy, the Romulan effort ultimately failed. A temporary alliance of Human, Vulcan, Andorian and Tellarite starships thwarted the drone-ship threat, prompting closer diplomatic relations that eventually led to the Coalition of Planets. (ENT: "Babel One", "United", "The Aenar", "Demons")
Relations between Romulans and United Earth deteriorated into open war in the following years. Beginning in 2156, the Earth-Romulan War was fought without any kind of personal or visual interaction between the two sides, preventing Humans or allies from fully understanding the nature of their opponent. Following a humiliating defeat at the Battle of Cheron in 2160, the Romulans agreed to a treaty establishing a Neutral Zone between the Star Empire's sphere of influence and territory that would fall under the jurisdiction of the soon to be formed United Federation of Planets. (TOS: "Balance of Terror"; TNG: "The Defector")
For the next century, Romulans isolated themselves from Human and Federation affairs. However, they remained very active within their own sphere of influence, fighting a number of campaigns, the nature of which is unknown.
In 2266, the Empire emerged from seclusion to test the strength of its old enemy. The Romulan Praetor ordered his finest flagship, a Bird-of-Prey led by an experienced commander, to violate the Neutral Zone and attack the Federation's observation outposts that lined the border in Sector Z-6.
Equipped with a cloak and a powerful new plasma torpedo system, the Bird-of-Prey easily annihilated its targets. A victorious homecoming, revealing Federation weaknesses, would have aroused the Empire to another war, but the marauding Romulan vessel was intercepted and defeated by the USS Enterprise before success could be reported.
During this era, many lower-ranking members of Romulan ship crews wore distinctive helmets, and were seen to stand, rather than sit, at their posts. (TOS: "Balance of Terror", TOS: "The Enterprise Incident")
In the face of Federation strength, the Empire vigilantly patrolled their side of the zone. Further encounters with Starfleet's Constitution-class starships were countered by squadrons of up to ten Birds-of-Prey to better the odds.
A brief but ultimately unsuccessful thawing in relations occurred in 2267 between the Romulan Star Empire, the Klingon Empire, and the Federation, with the founding of a jointly-managed colony on Nimbus III. Dubbed the "planet of galactic peace," the colony quickly became an embarrassing failure for all three governments, although regular meetings between representatives did take place at the colony for at least the next twenty years. (Star Trek V: The Final Frontier)
In 2268, a devastating polaric ion device explosion resulted in the destruction of a Romulan research colony on Chaltok IV. Following this incident, the Star Empire, together with the Federation, signed the Polaric Test Ban Treaty, banning research on polaric energy on both sides. (VOY: "Time and Again")
Also in 2268, Starfleet violated established treaties by ordering the USS Enterprise to pass through the Romulan Neutral Zone into Romulan space, where its crew confronted three Romulan ships, and returned with both a Romulan cloaking device and a captured Romulan commander. (TOS: "The Enterprise Incident")
After a technology exchange with the Klingon Empire, the first Romulan D7 class design battle cruisers began appearing along the zone by 2268, and were encountered by the USS Enterprise during its 2268 incursion into Romulan space. This Romulan-Klingon Alliance had come to an end by 2271, however, when the Romulans suffered a significant defeat at the Battle of Klach D'Kel Brakt (the "Briar Patch") at the hands of Klingon forces led by Kor. By 2292, Romulans and Klingons regarded each other as "blood enemies". (TOS: "The Deadly Years", "The Enterprise Incident"; TNG: "Reunion"; DS9: "Blood Oath")
By the mid-2280s, Romulans had lost their virtual monopoly as the region's best stealth technologists when cloaking devices began seeing service on Klingon vessels. (Star Trek III: The Search for Spock)
As the Klingons and Federation began détente in 2293, the Romulans attempted diplomatic subterfuge through Ambassador Nanclus' role in the Khitomer conspiracy – yet another failing effort. (Star Trek VI: The Undiscovered Country)
The Tomed Incident of 2311, an encounter between the Romulan Star Empire and the Federation that cost thousands of lives, led to the Treaty of Algeron, which forbade the Federation from developing cloaking technology. Subsequent to the incident, the Empire returned to a policy of isolation from the Federation. (TNG: "The Neutral Zone", "The Pegasus")
Though periodic, titular allies, the relationship between the Romulan and Klingon Empires was effectively a decades-long, "hot" and "cold" war of bitterly antagonistic cultures. In 2344, four Romulan warbirds attacked a Klingon outpost on Narendra III and destroyed the Federation starship that answered the distress call, the USS Enterprise-C.
Another outpost attack, the Khitomer Massacre of 2346, was facilitated by a secret Romulan alliance with the Klingon House of Duras. Whatever the intention of the offensives of the 2340s, Klingons came to regard the Romulans as "blood enemies", and the Federation-Klingon alliance strengthened. (TNG: "Yesterday's Enterprise", "Sins of the Father", "Reunion")
Later that year, the Empire was alerted by the mysterious destruction of border outposts on either side of the Federation/Romulan Neutral Zone. A Romulan Warbird was sent to investigate. Commander Tebok realised that the destruction was of different level of what the Federation would ever do. Meanwhile the USS Enterprise-D was also sent to investigate and establish contact with the Romulans after five decades. Captain Jean-Luc Picard proposed a cooperative investigation but was turned down. This incident, which unknown to both factions was caused by the Borg, caused the Empire to take notice, resuming an active, oppositional interest in their neighbor's affairs through the 2360s. (TNG: "The Neutral Zone")
The Klingon Civil War
- Main article: Klingon Civil War
Upon reemerging from their isolationist state, the Empire returned to their attempts to destabilize the peace of the quadrant, through efforts to sever the Federation-Klingon Alliance. In 2367, the Commander Sela abducted Lt. Commander Geordi La Forge, the chief engineer of the USS Enterprise-D. They brainwashed him in an attempt to use him to assassinate the Klingon Vagh, governor of Krios. The assassination was foiled by Enterprise personnel. (TNG: "The Mind's Eye")
Shortly thereafter, as part of the Empire's ongoing relations with the House of Duras, the Romulans became involved in the Klingon Civil War of 2367-2368. A convoy led by Commander Sela provided supplies to the Duras family to use against the forces of Chancellor Gowron. Their intervention was discovered by the Federation, and a Federation task force of twenty starships blockaded the Klingon-Romulan border, allowing Gowron's forces to prevail. (TNG: "Redemption")
The failure to end the Federation-Klingon Alliance did not stop the Empire from continuing their opposition to the Federation. In 2368, the Romulans attempted to gain a foothold within the Federation by using the underground Romulan-Vulcan reunification movement as a cover. Through Senator Pardek, a man trusted by the unificationists, Proconsul Neral captured Ambassador Spock, Captain Picard, and Lt. Commander Data. As Romulan forces moved into Federation space under a guise of peace, the three prisoners subdued their captors and warned Starfleet of the Romulan invasion. (TNG: "Unification I")
The underground unification movement continued even after this blow. In 2369, a prominent government official, Vice Procounsul M'ret, successfully escaped across the Federation border through the help of the unificationists. (TNG: "Face of the Enemy")
The Dominion War
- Main article: Dominion War
The emergence of the Dominion threat in 2370 led to the first thaw in the Empire's relations with the Federation. In 2371, an amendment to the Treaty of Algeron permitted the Federation use of a cloaking device for Gamma Quadrant operations, in exchange for intelligence gathered regarding the Dominion. (TNG: "The Neutral Zone"; DS9: "The Search, Part I")
As the Dominion War began in earnest, the Romulan Tal Shiar and the Cardassian Obsidian Order intelligence services launched a joint attack on the Dominion. The Battle of the Omarion Nebula in late 2371 was a catastrophic failure, devastating both Romulan and Cardassian forces. (DS9: "The Die is Cast")
As the Dominion finally gained a foothold in the Alpha Quadrant through the alliance with the Cardassian Union, the Romulans initially sent a fleet of vessels to Deep Space 9 to join the Klingons and Federation in anticipation of the imminent war. (DS9: "By Inferno's Light")
When war did not initially break out, however, the Empire signed a nonaggression pact with the Dominion, and planned to watch their neighboring rivals take the heavy losses, but a successful intelligence operation conducted by Captain Benjamin Sisko and Elim Garak of Deep Space 9 led the Empire to believe the Dominion posed an imminent threat, leading the Romulans to join forces with their erstwhile enemies in the war. (DS9: "Call to Arms", "In the Pale Moonlight")
Within hours of joining the war against the Dominion, the Romulans attacked fifteen bases along the Cardassian-Romulan border, quickly opening up a new front and drawing forces away from the Federation and Klingon lines. This initial Romulan attack marked the Federation Alliance's first inroads into Cardassian space in the war. (DS9: "In the Pale Moonlight")
Though bound by necessity, the new alliance was less than amicable. The mutual antipathy between Romulans and Klingons forced the Federation to serve as mediator, and all sides knew a battle for influence in the Alpha Quadrant would follow once the war ended. (DS9: "Tears of the Prophets")
In the last battle of the war, heavy losses were inflicted on the Romulans by the Dominion, Breen, and Cardassian forces. Their flagship was destroyed and their lines began to collapse. Captain Benjamin Sisko was ordered to help the Romulans, although they were ultimately saved by the Cardassians turning on the Dominion. In a proposed attack on Cardassia, the Romulans would have worked with the Cardassians in attacking the Jem'Hadar, although this battle was never fought due to the Female Changeling's surrender. Later, a female Romulan dignitary represented the Romulans at the official Dominion surrender ceremony on Deep Space 9. (DS9: "What You Leave Behind")
In 2351, the science vessel Talvath under the command of Dr. Telek R'Mor of the Romulan Astrophysical Academy was studying a unnamed micro-wormhole in Sector 1385 when it was contacted by the USS Voyager, a Federation starship stranded in the Delta Quadrant. After several exchanges, Telek R'Mor agreed to beam aboard Voyager, where he learned the ship was from twenty years in the future. Since Voyager's crew decided not to return and wait twenty years before they could resume their lives, it was decided to return Telek R'Mor with letters from the crew to thier families, to be delivered before Voyager was lost in the Badlands. Telek R'Mor unfortunately died in 2367, four years before the letters could be delivered, and was unable to deliver them. (VOY: "Eye of the Needle", "Message in a Bottle")
Voyager was also indirectly responsible for the failure of a Tal Shiar operation carried out by Rekar, with his aide Nevala, to steal the USS Prometheus in 2374. Voyager had sent their EMH, The Doctor, through the Hirogen communications network to the starship. Once aboard, The Doctor quickly learned that the crew had been killed and the ship was in Romulan hands. The Doctor, along with the help of the Prometheus' EMH Mark II, were able to incapacitate the Romulans aboard with neurozine gas shortly before three D'deridex-class warbirds arrived. A Starfleet task force arrived shorty thereafter, and after the destruction of one of their ships, the Romulans retreated. (VOY: "Message in a Bottle")
After the disappearance of a holographic representation of himself meant to be delivered to Voyager in 2377, Lieutenant Reginald Barclay first blamed the Romulans, citing that they've been interested in Voyager for years!. (VOY: "Inside Man")
The Reman Uprising
In 2379, Shinzon, a Human clone of Starfleet captain Jean-Luc Picard, gained support from the Romulan military and eventually planned to take over the Romulan Star Empire by appointing himself as Praetor and assassinating the entire Romulan Senate, except Senator Tal'aura, who had been later allied with Shinzon along with Commanders Suran and Donatra.
When the Federation starship USS Enterprise-E arrived at the Romulan system for peace talks between the Federation and the Romulans, Shinzon delayed. This delay was part of a ruse, taken due to his accelerating deterioration, so that he could capture Captain Picard and use him to return to health.
Unfortunately for Shinzon, Picard successfully escaped, and Shinzon pursued him to the Bassen Rift where Picard again eluded capture with the assistance of Romulan reinforcements. Commander Donatra had realized that Shinzon didn't just plan to attack the Federation, but actually planned total annihilation for Humanity and other species. Ultimately, Shinzon was killed in hand-to-hand combat with Picard aboard his starship, the Scimitar, just before the ship's thalaron radiation generator was overloaded by Commander Data, who sacrificed his life to save the Enterprise.
In the aftermath of the Battle of the Bassen Rift, the newly-minted Captain William T. Riker of the USS Titan, led a task force to the Federation-Romulan border to engage direct negotiations for a final peace between the two sides, which would hopefully lead to a greater understanding and a new era of peace in the Alpha and Beta Quadrants. (Star Trek Nemesis)
In 2387, a nearby star went supernova and threatened Romulus with annihilation. Ambassador Spock pledged to save Romulus by detonating red matter into the star, turning it into a black hole. However, he arrived too late and the star was consumed by the red matter singularity only after its supernova destroyed Romulus. (Star Trek)
In an unknown, alternate future timeframe, the Klingon Empire entered a expansionist phase that brought it and the Romulan Star Empire to war. The Romulans were defeated, with their territory occupied by the Klingons. This timeline was avoided by the actions of Jean-Luc Picard. (TNG: "All Good Things...")
In an alternate timeline caused by the USS Defiant accidentally sending three officers into Earth's past, the Federation never came to be. When the crew of the Defiant scanned for Federation and alien outposts all they could find were Romulan transmissions detected in the vicinity of Alpha Centauri in 2371. (DS9: "Past Tense, Part I")
In the alternate reality created by Nero, by 2258 the Federation had become aware of the Romulans' common ancestry with the Vulcans, and all three of their dialects had become understandable to at least one communications officer. (Star Trek)