2154年より以前から、ヴァルカンとロミュランの再統一運動が市民レベルで高まりつつあった。同年、ヴァルカン最高司令部のヴラス長官の指揮の元、ヴァルカンはアンドリアがズィンディ技術に基づく新兵器を開発しているという理由で、アンドリアに対して先制攻撃を加える計画を推し進めた。これはヴァルカン星上の地球連合大使館爆破事件をきっかけに地球連合及び宇宙艦隊が介入し、この爆破事件の首謀者がヴラスであったことが発覚した。しかし、この時点ではヴラスの裏にロミュランの工作員が関わっていたことはまだ明らかにされていなかった。（ENT:狙われた地球大使館、ENT: 陰謀の嵐、ENT: バルカンの夜明け）
その年の後半、ロミュランは完全自動化された無人艦を用いて、アンドリアとテラライトの二国間に戦争を引き起こすために行動を開始した。しかしながら、この計画は当初の目的とは全く逆の方向へと進んでしまった。後にバベル危機と呼ばれるこの事件で、地球連合のジョナサン・アーチャーの仲裁により、それまで対立関係にあったヴァルカンとアンドリア及びテラライトが一時的に同盟を結び、協力してロミュランに対抗してきたのであった。（ENT:バベル1号星、ENT: ロミュランの陰謀、ENT: 氷窟の民）
|十字軍 • アメリカ独立戦争 • 南北戦争 • 第一次世界大戦 • 第二次世界大戦 • ベトナム戦争 • 地球冷戦 • 優生戦争 • 第三次世界大戦 • 地球・クジン戦争 • 地球・ロミュラン戦争|
- ↑ 劇中で語られたヴォーカド及びガロンドン・コアの戦いはこの戦争に関連していた可能性があるが、具体的な時期が語られていないため依然不明のままである。しかし、シャロンの戦いの以前に発生したと思われるため、ロミュラン戦争中の戦闘である可能性が高いとされている。
The details of the Earth-Romulan War are still somewhat uncertain with little canonical evidence. As Spock said about the war, "As you recall from your histories, this conflict was fought, by our standards today, with primitive atomic weapons and in primitive space vessels which allowed no quarter, no captives. Nor was there even ship-to-ship, visual communication; therefore, no human, Romulan, or ally has ever seen the other. Earth believes the Romulans to be warlike, cruel, treacherous... and only the Romulans know what they think of Earth." The exact dates for the war and several details above are derived from an illegible viewscreen graphic seen in the Star Trek: Enterprise episode In a Mirror, Darkly, Part II. The graphic comprised of data points from an outdated version of the Star Trek Chronology.
Prior to this, a behind-the-scenes newspaper clipping titled It's Federation Day!, which was among the collection found in the Picard family album from スタートレック：ジェネレーションズ, dated "11 Oct. 2161," stated that the war ended "a little more than a year" prior to the final declaration of peace and the foundation of the Federation. This source also stated that the war ended with the Battle of Cheron in early 2160. This information did not appear on screen.
Some evidence indicates that Earth itself commenced the conflict, as Data remarked in "The Defector" that "in their long history of war, the Romulans have rarely attacked first... they prefer to test their enemy's resolve." In other words, Earth may have been provoked into acting first, either intentionally or possibly inadvertently. However it may have been one of the few occasions that the Romulans did attack first, such as the events that transpired in "Balance of Terror."
The Star Trek: Enterprise series finale These Are the Voyages... presents the formation of an alliance that in the words of Deanna Troi, "will lead to the founding of the Federation." It is unclear whether or not this "alliance" was the one spoken of between Earth and its allies during their opposition against the Romulans as Troi made no mention of the Earth-Romulan War itself.
There are several points of ambiguity involved with this time period. For more information on those topics please visit the following links: cloaking device, Romulan Bird-of-Prey (22nd century) and Romulan Bird-of-Prey (23rd century).
In the Spaceflight Chronology, more detailed accounts are given of the events that led to the war and the war itself. 14 years prior to the actual war, the Romulan Star Empire began a campaign of attacks disguised as random space pirate activity. The first of these attacks was the hijacking of the ore freighter USSMuleskinner. Two years later the Delta VII outpost was attacked. Five years after this, the commercial transport Diana was attacked with its cargo stolen and crew killed. Four years prior to the actual war, the reports on piracy attacks were for the first time percieved as possibly being organized. Three years before the war, Starbase 1, in Sector 3A, was destroyed by what was still believed to be an act of space piracy. Romulans were for the first time identified as the culprits. All transports were given military escorts at this time. Two years before the war, the Romulan threat became apparent. A year before the war, Romulans made a number of hit and run attacks to several sectors. The actual war began when a full Romulan invasion fleet advancing on the Rigel system was encountered by the destroyer USS Patton. In the second year of the war, Romulans managed to destroy a mining complex in Sector 5B. A Romulan armada was completely destroyed in Sector 7. In the third year of the war, Romulans attacked Alpha Omega. Victory in the war was credited to advanced ship-mounted laser weaponry, developed only two years before the war as a response to the destruction of Starbase 1.
The dates of these events in the timeline of the Spaceflight Chronology differ from the canonical timeline. Also, the major difference between these accounts and canonical accounts is that the war is stated to have been fought by the Federation, not the allied forces as was established later on in the viewscreen graphic from In a Mirror, Darkly, Part II.
In the Star Trek Star Fleet Technical Manual, Franz Joseph offered a one-page version of the subspace peace treaty that concluded the war. It was signed by "Christofur Thorpe" for Earth and "Praetor Karzan" for the Romulans.
The fourth season of Enterprise began to show the very early beginning of the coming conflict with the Romulans. Had Enterprise been renewed for a fifth season, it would have continued to show the whispers of the Romulan War and eventually allow the conflict to be realized on screen. Additionally, Rick Berman and Brannon Braga considered making Future Guy a Romulan. 
The non-canon Star Trek: Enterprise novel Last Full Measure suggests that the Federation was born from the conflict. The Last Unicorn Games book Among the Clans: The Andorians makes reference to a series of Earth-Romulan wars, taking place during roughly the same time as the single canonical war.
In the 2006 game Star Trek: Legacy it is revealed that a Vulcan scientist named T'Uerell supplied the Romulan Star Empire with metagenic weapons to use against Earth colonies. Earth used the radiation from nuclear reactors in the Tyburn system to halt a large Romulan force from reaching Earth – explaining the use of atomic weapons.
The non-canon series by Michael A. Martin starting with The Good That Men Do covers the events going from These Are the Voyages... through the Romulan war. It starts with the apparent death of Trip so he can spy on the Romulans attempt to create a warp-seven capable ship. According to the series, the Romulans started the war out of fear of the birth of the Coalition of Planets. The Romulans started the war by attacking proposed member Coridan (thus delaying their entrance into the alliance until the Original Series and then by attacking a number of Coalition ships using a telecapture device and destroying the Kobayashi Maru as seen in the novel Kobayashi Maru. The novel The Romulan War - Beneath the Raptor's Wing details the first year of the war.