In Surak's time (somewhere in the 4th century AD, Earth calendar), Vulcan was invaded by Etoshan pirates. These pirates initially came with messages of peace, only to kidnap or slaughter large numbers of Vulcan citizens. Up until that point, Vulcan was still a fractured planet of many families and clans. Under threat from the Etoshans, Vulcan united to drive off these invaders.
After the Etoshans were gone, Surak argued that peace and logic was still a possibility. One of his students, S'task, countered with the evidence that the universe was hostile and should be met with force. Neither side wished to destroy Vulcan itself over this difference of opinion, so it was gradually decided by all that S'task and his followers should leave Vulcan and seek a new homeworld.
In a process that could have bankrupted Vulcan, 16 huge interstellar ships were designed and built over a period of more than two decades. Each ship was designed to carry a passenger compliment of approximately five thousand people. Although they had no warp drives, they could get up to near-light speed with the use of deadly (for the person doing it) psionic techniques.
Starting almost from scratch, and while waiting for their ships to be completed, these soon-to-be exiles created an entirely new language and came to call themselves “the declared” or, in their language, Rihannsu. Over time, they also developed their own Romulan religion. Not everyone who embarked upon these generation ships actually wanted to go. Some were pressured into it by others, for various reasons. Traces of this, especially resentment between various families and clans, persist to this day.
The launch date is officially known by Romulans as “the Sundering”. The date is sometimes celebrated and sometimes mourned, though it is not an official holiday.
At the beginning of the trip, multiple failures and malfunctions plagued the vessels. There were several crop failures and mild famines, though few people died of anything rare maintenance accidents. Seven ships were lost in the event horizon of a black hole. Two more ships were lost to psychic “vampires” at the planet Iruh. Another two ships were lost at separate times due to unreparable drive system malfunction. Plagues, perhaps mutated by the drive radiation, began to kill off fifty to seventy percent of the passengers on the remaining ships.
Of the sixteen ships that left Vulcan, only five survived to reach ch'Rihan and ch'Havran. Although ch'Havran, or Remus, was poorer in resources than ch'Rihan, or Romulus, both planets were infinitely richer than the Vulcan that they had left behind.
Despite the unprecedented riches, there were still squabbles among the eighteen thousand surviving Romulans over who would get the best pieces of land. Some did not want to leave, electing to stay aboard ship as long as possible. This group became known as the Ship Clans. The rest held a lottery, which even today some claim was rigged, to decide who went where. What is known is that most of the reluctant travelers ended up on the worst parts of ch'Havran and not on the lush world of ch'Rihan.
Due to low number of survivors, drastic measures were taken to ensure that the Romulan people wouldn't die out. Although no law was ever written, even the poorest of women were encouraged to have no fewer than four children. Large families were the norm, with some of the more affluent Houses seeing it as their duty to “adopt” those who could not afford families of their own. Over time, these adopted members grew to be the servant and slave class of the wealthier Houses.
Because of the small gene pool available, genetic manipulation and IVF treatments were bought by the wealthiest in order to increase their numbers and avoid their offspring having any inheritable disabilities. Modern Romulans have a reputation for destroying any infant that is obviously imperfect, but they like to forget that this wasn't always so and most do not even know their own history.
For a time, it was “fashionable” to exile political dissidents and otherwise undesirable citizens to the most hostile, inhospitable, and otherwise uninhabited locations on the Eastern Continent of ch'Havran. Deformed or otherwise supposedly weak children were also sent to these places, to be cared for by the political prisoners and their overseers. This was not completely unlike the nearly-forgotten Earth custom of “exposing” an unwanted infant and leaving it to die. Those who survived this treatment were still considered less than full Romulans, but had proven their worth.
When precious stones like rubies and dilithium were discovered under these hostile reaches, these less-than-Romulans were the perfect choice to mine the ore. Over time, exposure to dilithium radiation, hard work, lack of sunlight and so forth mutated these beings into the race known today as Remans.
Even while trying to build their numbers, the Romulans were busily and cheerfully engaged in killing each other off. Of the eighteen thousand who arrived at the homeworlds, five to six thousand of those died in the first decade after planetfall. Fresh water was often a contested resource, although there was no shortage of lakes and streams for the Romulans to use. Each powerful House reckoned power and wealth slightly differently and they were only too glad to fight for an even better position.
Modern Romulan warfare is usually focused on yikh, or outsiders, rather than on each other. However, power struggles and quiet assassinations are still extremely common among the upper classes.
After centuries of isolation, the Romulans had nearly forgotten that there were aliens in the universe. Vulcan was a distant memory. The Great Ships had been slowly abandoned by the Ship Clans and, eventually, fell from orbit. Only small interplanetary shuttles remained, to ferry goods between the two worlds.
The memory of the rest of the galaxy was reawakened around fifteen hundred years after planetfall, when the Federation ship USS Carrizal arrived in the system. Upon arrival, it began broadcasting messages nearly identical to the ones the Etoshans had used. Obviously, the only response was to hastily build ships for a brutal, savage and unrelenting interstellar war.
Although the Federation had better technology, Romulans had an advantage of numbers and sheer tenacity. Captured Federation ships helped provide new technology, such as warp drive, to the Romulans. Although neither side was able to overwhelm the other, the Romulan tactics eventually forced the Federation to sue for peace.
Following the war, the Romulans entered an age of expansion. Although the Neutral Zone Treaty had established a boundary on the Federation side, there were plenty of star systems on the Romulan side of the zone to conquer. Although native populations were exceedingly rare in their section of space, those that were found were quickly subjugated and forced to serve the Romulan people. Uninhabited planets were made inhabitable and colonists were shipped in.
This expansionist period and the lack of serious infighting saw the Romulan population nearly tripled in a matter of just a couple decades. During this period, contact was made with the Klingons. Although distrustful, both empires initially saw a kinship in the other. In exchange for not conquering Klingon border worlds, the Klingons would provide the Romulans with badly needed ships. Or so the Romulans claim. In the end, the tentative treaty crumbled and left behind only an uneasy wariness.
Although the Romulan Star Empire had not existed when the treaty with the Federation was signed, such an entity was in place by the time they developed the cloaking device. Various machinations were tried, the most audacious of which resulted in the First Romulan Civil War. After being thwarted by Kirk, Picard, and other Starfleet captains, the Romulans finally agreed to the signing of the Treaty of Algeron.
During the Dominion War, the Romulans somewhat reluctantly sided with the Federation. Although siding with their sworn enemy was considered to be a poor choice, even the most traditional realized that defeat and destruction at the hands of the Dominion would be much worse. With the end of the Dominion war, this alliance was broken and the Romulans once again viewed the Federation with hostility and suspicion.