The Earth–Romulan War was a major conflict fought between the United Earth and the Romulan Star Empire in the fictional Star Trek universe.
Humanity first made contact, indirectly, with the Romulans in 2152, when Enterprise NX-01 strayed into Romulan space and became ensnared in a minefield. The Romulans nearly destroyed the ship before it could escape. Earth had never heard of the Romulans, and Vulcans knew of them by reputation, but in fact the Romulans knew a great deal more about both of them. As is now common knowledge, the Romulans are an ancient offshoot of the Vulcan people. Their ancestors left Vulcan during the Time of Awakening because of their opposition to the teachings of Surak. They wandered through space on meager vessels until settling on a world which human cartographers called Romulus. The Romulan ships were exhausted, and were disassembled to construct the new settlement. Most of their then-advanced technology was lost.
It would take the Romulans until the early 22nd century to regain even limited warp flight. The Romulan region of space was relatively poor in resources, and expansionist forces soon urged the Romulans to push outward. During this time, Romulus 'rediscovered' its Vulcan cousins, although they remained largely anonymous. Romulans also first encountered humanity. Aries, an old Earth low-warp explorer, wandered into Romulan-held space in 2148. The ship was captured, its crew interrogated and reportedly dissected. Similarly, the ship itself was broken up and analyzed. Aries was reported missing in the following months, but no sign of it was ever recovered by the Earth Starfleet.
In subsequent years, human shipping would suffer a building number of anonymous attacks that have since been attributed to Romulan pre-war hostility, culminating with that first official encounter with the Romulans by Enterprise in 2152. The configuration and level of technology of the vessels encountered by Enterprise contrast starkly with the vessels in use by the Romulans before, during, and after the war. They even had practical cloaking technology over a full century before the Romulans employed it for widespread use. These ships were testbeds for new technology, and the destruction of the lead ship, the Praetor Pontilus, in a catastrophic antimatter failure caused by her cloaking system, ensured that it would be decades before the Romulans would possess a successful cloaking device. The Romulans, like humanity with the NX-class starship, had stumbled across technology ahead of their time that would ultimately take decades to perfect.
After the initial encounter of 2152, Starfleet made a concerted effort to learn everything they could about the Romulans and establish diplomatic contact with them, but to no avail. In 2156, humans established their first starbase in the Berengaria system, designated Station Salem One. The paranoid Romulans, already suspicious of human expansion and contact with the Vulcans, were afraid Salem One would be an outpost from which to attack their interests and claims on resource-rich worlds. These, and a number of other 'incidents' with the Romulans, including ambushes of Earth vessels and other simply unprecedented attacks and civilian atrocities made war almost inevitable.
The Romulans utterly annihilated Salem One, killing hundreds and destroying the Starfleet ship Endeavour, despite the arrival of Enterprise, NX-01, on the scene. The Terra Prime movement, which believed all aliens should be expelled from Earth outposts, blamed all alien races and called for swift retribution. The Romulan fleet, divided into attack wings, proceeded to raid Earth bases and destroy Earth ships and communication buoys. Earth pressed for support from other members of the Coalition of Planets, but none were prepared to fight the Romulans. Indeed, the Coalition had been proven impotent almost instantly after its creation and soon dissolved. Exhausted of diplomatic alternatives, Earth prepared to go to war against the Romulan Star Empire virtually alone. Only the Vulcans continued to stand with humanity, but, in light of their recent societal re-alignment towards near pacifism, in only a limited role.
Earth was truly ill-equipped for the bloody conflict that lay before them. The Romulan fleet, while relatively primitive, was well-organized and capable, while Starfleet had barely gotten into deep space. The fleets had different focuses. Starfleet had long been building ships of peaceful exploration and patrol, and despite the Xindi threat of 2153, continued to do so. Starfleet had also only begun building matter-antimatter-powered starships within the last two decades. Fusion-powered ships may have been acceptable for short-range exploration, but were largely unsuitable for interstellar combat. Three more under construction NX-class ships were rushed into the launch stage with virtually only warp and weapons in operation, with mixed service results. Romulan ships, however, were built for conquest. Romulan warp sleds could deploy entire squads of low-velocity birds of prey within striking distance of Earth targets from one carrier. Indeed, this tactic devastated several human outposts. Meanwhile, long-range warbirds plundered and destroyed human freighters and outposts across the known regions, crippling Earth's supply infrastructure before the war had even truly began.
Vulcan provided some relief, but Starfleet's production of NX-class and other such pre-war ships was almost hopelessly behind due to supply shortages. Even if Starfleet had the resources, NX-class ships and her cousins could take up to three years to be built from scratch. Starfleet engineers were sent back to the drawing boards to design more efficient vessels. Starfleet needed even smaller, dedicated fighting ships capable of combating the agile Romulan birds of prey, and thus the so-called Cylinder Ship Program was born. "Flying submarines" as some called them, little more than simple cylinders with warp nacelles, filled this particular niche, with existing NX-class ships becoming the equivalent of capital ships, being the only ships with true ranged fighting capabilities.
The "flying submarines," such as the Farragut and Conqueror classes, were little more than guns with engines. Their designs were derived from the lineage of Dyson-Yoyodyne Corporation's DY series, made popular in the first decades of Earth's warp flights. On these ships, even crew quarters seemed an afterthought. Space was at a premium, with no room for quarter or compromise. Supplies were rationed precisely to avoid waste. Starfleet captains by necessity could not take prisoners. Weapons primarily consisted of fusion torpedoes and pulse cannon lasers. Most were only equipped with the most bare hull armor. Their warp drives were also quite limited, and quite dangerous. What the ships possessed in speed and agility, they lost in durability. Classified Starfleet reports of the era suggested that "Command should be prepared to accept a minimal ten percent loss of ships due to catastrophic reactor failure alone." However, their crews, usually approximately a dozen per ship, knew the risks all too well even without being briefed on them. Many didn't make it home.
Despite a valiant effort by Earth and her Vulcan allies, humanity continued to suffer heavy casualties for the next two years. Romulans employed "primitive" nuclear weapons, which proved surprisingly effective against the polarized shielding of Earth vessels, resulting in profound radiation poisoning and death among their victims. Ground battles were practically rendered moot, as nuclear planetary barrages of enemy strongholds, even destroying their own troops, seemed to be Romulan modus operandi. During an early skirmish, a Starfleet squadron defeated a superior Romulan attack force near Delta IV. Moving to capture the disabled Romulan ships, the Earth ships were caught off guard when the Romulans suddenly destroyed themselves, taking a number of Starfleet ships with them. It soon became standing Starfleet orders to destroy disabled Romulan ships rather than allow them to engage in such kamikaze detonations. Visual encounters with Romulans were non-existent, and such that, even by the war's end humans still did not know what Romulans looked like.
From the initial spear the Romulans captured into Coalition space, Earth gained momentum in re-taking its former holdings, hopping from planet to planet. Human shipbuilding slowly began to recover. Earth continued to prod its fellow Coalition members to join Earth in the fight, and finally, by 2159, the Andorians, Tellarites, Denobulans, Axanar, and most of the rest of the other Coalition species had agreed. All of these races had suffered casualties and shipping losses during the war at the hands of the Romulans. The entry of Earth's allies into the war marked the turning point in the war. The combined fleets were able to put a stop to most of the Romulan raids on Earth outposts and freighters, enabling Starfleet to begin to recover from the supply shortages. Captain Jonathan Archer of Enterprise is recorded as being instrumental in negotiating the entry of many of these species into the fight, particularly the Andorians.
In late 2159, Andorian intelligence indicated the Romulans were preparing to mass a large task force near their border to strike Earth in what would be a devastating final surprise attack. However, the allied forces were able to mobilize their fleets and launch a critical preemptive strike before the entire armada was massed, destroying almost the entire Romulan armada. A small Romulan force escaped and made their way towards Earth, intent on finishing the fight once and for all by bombarding Earth with nuclear weapons. The allied fleet was able to stop the assault near the orbit of the Plutonian moon, Charon. The Romulan fleet was left virtually crippled after their flagship was destroyed. Surviving ships were destroyed or self-destructed on site. Subsequently, the Romulan Empire agreed to an armistice. A treaty was subsequently negotiated over subspace, establishing a Neutral Zone between the Romulan Empire and the Coalition worlds. The Romulan Empire entered a sort of self-imposed isolation for the next hundred years in defeat. Earth casualties were of a caliber not seen since the last world war.
The unfortunate irony of this war was, of course, that it saw birth to the Federation. Fighting together proved that the various former coalition races could truly work effectively together, and Earth's first stellar war had served to help unify her people. That, combined with a desire to prevent the threat of the Romulans or anyone else from endangering them again, prompted the governments of Earth, Vulcan, Andoria, and Tellar to found the United Federation of Planets in 2161.