In the early 2150s, United Earth began establishing itself as an Alpha Quadrant power through diplomacy and also military advancements. For instance, they managed to end a conflict between the Andorians and the Vulcans that was all about a moon. Wow! They also helped to enhance relations with the Tellarites.
The Romulans were not all good with that in fact they saw this as an unacceptable evolution of events and feared that an alliance between these powers might be formed. This led them to start a very secret and stealthy campaign in order to destabilize the region and maybe even starting a conflict with these powers.
At some point before 2154, the idea of a reunified Vulcan and Romulan race became quite popular but this had to wait since Vulcan High Command under the leadership of V’Lass planned a preemptive strike against the Andorian Empire because it was believed that the Andorians were developing weaponry based on the Xindi superweapon.
Another incident which would be the bombing of the United Earth embassy pushed the Romulan reunification plans back further. Although they still stayed in touch with V’Lass, his attempts to start a war between the Andorians and the Tellarites were prevented thanks to Jonathan Archer and the crew of the NX-01. Instead of dividing the local powers even further they now began working together more closely.
In 2155 the work the Romulans did began to have an impact on the local powers. Just not they wanted it. A coalition between the Earth, Vulcan, Andoria and Tellar was formed. Other species were as well involved but we just stick around with these.
Because of the formation of the Coalition of Planets was seen by the Romulans as a provocation. The open war began between Earth and Romulus, in 2156 and went on to 2160.
The Earth-Romulan War was described by Spock as being fought with primitive nuclear weaponry which contradicts with what we see on Enterprise. It is also described that Romulan ships had a Bird of Prey painted on the ships ventral hull which never was shown in Enterprise.
Casualties during this war were high on both sites as for United Earth people like Captain Stiles and several other of his family members died during it. According to memory alpha and what we hear on screen, MACO teams were not an unusual site aboard Starfleet ships. As we know that Balthazar Edison was a MACO.
The war concluded with the “Battle of Cheron” in 2160 and is considered to be a great victory for Earth and her allies. Ending with a humiliating defeat for the Romulans and the formation of the United Federation of Planets the Earth-Romulan war is an important part of Star Trek history that we unfortunately never saw on screen.
However, we know much about its aftermath. After the “Battle of Cheron”, both sides negotiated terms via subspace. The result of this was the formation of the neutral zone and the creation of several outposts along with it. Some say it served its purpose to well because there were no contacts with Romulans mid 23rd century. Two centuries after the war the Romulans were back. However this time a Starfleet´s ally in the Dominion war.
Watch the Star Trek Lore Breakdown: The Earth-Romulan War video below