The Earth-Romulan War, also known as the Romulan War, was a major interstellar conflict fought from 2156 to 2160 between the forces of United Earth and those of the Romulan Star Empire. Earth was assisted by its allies Vulcan, Tellar, and the Andorian Empire.
In the early 2150s, Earth began to make impressive diplomatic and military advancements in the Alpha Quadrant. Major conflict between the Vulcans, Andorians, and Tellarites had for the most part ended thanks to diplomatic intervention by United Earth and the starship Enterprise (NX-01), under the command of Captain Jonathan Archer.
The Romulans saw this new-found peace in the region unacceptable, and thus began a carefully crafted campaign to destabilize the region and maintain the status quo of conflict between the quadrant's major powers.
At some point prior to 2154, a reunification movement began to become popular among some Romulan and Vulcan citizens. In that year, the Vulcan High Command, under the leadership of Administrator V'Las, planned to launch a preemptive strike against Andoria, citing that the Andorians were developing weaponry based on Xindi technology. Additionally, the Earth embassy on Vulcan was bombed prior to the attempted execution of these plans, prompting involvement by United Earth and Starfleet. Although V'Las's plans were halted and his involvement with the bombing was exposed, his connection with a Romulan operative who had been working on Vulcan with V'Las for several years, remained secret.
Later that year, the Romulans continued their campaign of destabilizing the Alpha Quadrant by sending an automated Romulan drone-ship|drone ship to attack Andorian and Tellarite targets, with the hopes of provoking a war between the two powers. Unfortunately for the Romulans, this had the opposite effect of what they were hoping to achieve; instead of destabilizing and dividing the region, the attacks on United Earth, Vulcan, Andoria, and Tellar resulted in a successful allied effort to stop the drone ship and return peace to the region.
By 2156, United Earth and the Romulan Empire entered a state of total war that would continue until 2160.
In 2266, Spock categorized the weaponry generally used in the Earth-Romulan War as atomic in nature, and fairly primitive at that. He also characterized the ships in use as primitive, and designed in ways that did not permit quarter or captives.
Romulan starships were painted to resemble a "bird of prey", a tradition kept through the mid-23rd century. Ship-to-ship visual communication between Earth-allied ships and Romulan-allied ships did not take place during the war or during the peace negotiations.
Among the casualties during the course of the war include several members of the Stiles family, including one Captain Stiles.
The Battle of Cheron in 2160 is generally considered the decisive battle in the Earth-Romulan War. Both sides recognized the major victory achieved here by Earth and its allies as a humiliating defeat of the Romulan military. This event would establish long-term political effects within the Romulan Empire lasting well into the 24th century.
Despite the Romulan's 2154 campaign of destabilization and the war that followed, Earth and Romulus learned little of each other in the 22nd century. In fact it would be over a century before any Human or Romulan would even see what their enemy looked like.
Following the Battle of Cheron the two sides negotiated a treaty via subspace radio. Among other things, it established a Neutral Zone, entry into which by either side would constitute an act of war. Earth set up at least eight Earth Outpost Stations on asteroids along its side of the Neutral Zone to monitor Romulan activity.
From a 23rd century Human perspective, the Neutral Zone represented an area "between planets Romulus and Remus and the rest of the galaxy..." Some Humans still harbored resentment over the war after over a century of peace, including Lieutenant Stiles of the USS Enterprise NCC-1701.
Despite these Human attitudes, until 2266 the Neutral Zone served its purpose and the treaty had remained unbroken. In fact, one might say it served its purpose too well, as there were limited or no contacts between Earth and the Romulan Star Empire from the conclusion of the war until the mid-23rd century.