Earth-Romulan War

The Earth-Romulan War was an interstellar conflict between United Earth and the Romulan Star Empire from 2156-2160. The conflict was the genesis of a mutual defense pact between United Earth, the Vulcan Confederacy, the Andorian Empire, and the Tellarite Union that would lead to the formation of the United Federation of Planets following the war's conclusion.

The Battle of Cheron in 2160 is generally considered the decisive battle that marked the end of the conflict, in which a combined Human/Vulcan/Andorian/Tellarite fleet routed a Romulan invasion fleet intent on attacking and occupying Earth. Following the Battle of Cheron, Romulan forces withdrew to pre-war territory and sued for peace.

Peace negotiations were conducted via audio-only subspace transmissions and established the Romulan Neutral Zone. Following the signing of the peace treaty, the Romulans withdrew to complete isolation until reemerging in the year 2266.

During the conflict, very little was known about Romulan physiology or culture. Unbeknownst to the other members of the alliance, the Vulcans of the time strongly suspected that the Romulans were descendants of their own race. But without confirmation and fearing the damage such a revelation would do to the nascent alliance, the Vulcans successfully kept this suspicion amongst themselves until it was confirmed over 100 years later. Many historians consider the question of how events might've played out differently had the Vulcan/Romulan relationship been revealed during the Earth-Romulan War to be the greatest "what if" question in history.