The Earth-Romulan War was one of the earliest major conflicts in the Star Trek timeline, but it had an impact that lasted for centuries in the story. Taking place during the events of Star Trek: Enterprise, the Earth-Romulan War broke out not long after Earth started to become a major player in the Alpha Quadrant, and it was the defining moment of the planet's first incursions into interstellar politics. Though the actual war was rather brief, the outcomes were felt for centuries after fighting officially ended, and it was one of the few conflicts that never truly ended between the belligerent parties.
Unlike other early scrapes that Earth ships found themselves in during the 2100s, the Earth-Romulan War was waged between a combatant seeking peace and one who would never surrender. While all wars in the Star Trek timeline were abhorrent in their own ways, the battles with the Romulans were particularly harsh because of the rigid nature of the Romulan Star Empire and their unquenchable desire to hold dominion over the entire galaxy. Despite the loss of life that was suffered during the war, some good did come out of the conflict, and it managed to shape the trajectory of the Star Trek timeline in positive ways as well.
Why The War Started
The efforts of the Enterprise's Captain Jonathan Archer during the 2150s saw Earth become an important figure in Alpha Quadrant business, and they expanded their alliances with planets like Andoria and Tellar Prime. The connection between Earth and Vulcan had long been established, but even tensions between the Vulcans, Andorians, and Tellarites began to ease with Earth's help. The Romulan Star Empire aimed to control all four quadrants of Star Trek's galaxy, and the alliances forming in the Alpha Quadrant between the four planets were not conducive to their plans for domination.
A Romulan/Vulcan unification movement began to take a foothold in the Vulcan High Command, and sympathizers planned an attack on Andoria under the pretense of the latter using Xindi weaponry. The plan was thwarted, but the Vulcan embassy on Earth was bombed by terrorists in 2154 during the events of Star Trek: Enterprise season 4, episode 7, "The Forge." The final leg of the Romulan destabilization plan came when drone ships were deployed to instigate a war between the Tellarites and Andorians, but it only served to bring the former enemies together to fight a common enemy. The four offended worlds eventually formed the Coalition of Planets in 2155.
The Outbreak Of War And Major Battles
The number of ships in Starfleet in the 2150s was much lower than it would be in later eras, but Earth did not fight alone against the Star Empire. A state of war was officially declared in 2156, and it kicked off five straight years of almost constant conflict that saw the Coalition of Planets take on the resourceful and unrelenting Romulans in vicious combat. Due to the Romulans operating a communications blackout, the Star Empire did not speak with its enemies, which made negotiations impossible. As such, battles were fought to the death, and the possibility of surrender was completely removed from the equation.
Little was revealed about the battles of the war in canon, but other Star Trek media hinted at important moments from the conflict. Chulak was a Romulan Commander who was mentioned in one of Star Trek's scariest episodes, Voyager season 2, episode 23, "The Thaw," and his losses at the Battle of Vorkado and Galorndon Core were major moments in the Star Empire's history. Though no in-canon source fully confirmed when they took place, they most likely occurred during the Earth-Romulan War. Federation officers would encounter Romulans on Galorndon Core again in Star Trek: The Next Generation season 3, episode 7, "The Enemy," in 2366.
The Battle Of Cheron
Though information about individual battles was scarce in canon, the Battle of Cheron was specifically mentioned on several occasions as a major turning point in the war. The planet was, according to the apocryphal source Star Trek: Star Charts, located within the Romulan Star Empire and was home to the native Cheron people. Despite possessing some of Star Trek's deadliest starships, the Romulans were served a decisive loss at Cheron from the Coalition of Planets, and they were unable to recover from their defeat. Though details of the actual battle were left unknown within canon sources, it summarily ended the war and led directly to negotiations.
The War's Outcome
Following the loss at Cheron, the Romulan Star Empire opted to negotiate of peace with the Coalition of Planets. They conducted their negotiations over subspace radio which meant that few if any ever actually saw the face of a Romulan during the war. In fact, despite half a decade's-worth of intense fighting, Earth forces learned next to nothing about their elusive enemies, and that was certainly a strategic choice by the Romulans. The of the treaty involved the foundation of a demilitarized Romulan Neutral Zone which would persist throughout almost every era of the Star Trek timeline.
Both sides agreed that broaching the Neutral Zone would be an act of war, and between the Romulan Star Empire and the rest of the quadrant was nonexistent for a century. However, in 2266 during the events of The Original Series season 1, episode 14, "Balance of Terror," a rogue Romulan ship began attacking Federation outposts near the Neutral Zone, and the efforts of Captain James T. Kirk thwarted another war. Romulan encounters would grow in frequency throughout the 23rd and 24th centuries, and the Star Empire would even assist the Federation Alliance during the events of the Dominion War in the 2370s, but the peace was short-lived.
The Founding Of The Federation
Perhaps the only positive to come out of the five years of terrible conflict was the eventual founding of the United Federation of Planets. One of the good things in Star Trek: Enterprise's finale was that it chronicled the founding of the Federation, and it did so out of the bonds formed during the war. A year after the conclusion of the Earth-Romulan War, the of the Coalition of Planets further solidified their pact by being the first four planets to the United Federation of Planets, and their individual identities helped craft the identity of the Federation in Star Trek as well.