The Earth-Romulan War (sometimes called just The Romulan War) was a major interstellar conflict that arose between the Romulan Star Empire and United Earth in 2156 and ended in 2160. United Earth would become allied with the Coalition of Planets, which included Vulcan, Tellar, and the Andorian Empire.
During the 2150s, United Earth had made great diplomatic strides at bringing peace and cooperation between the Vulcans, Andorians, and Tellarites. The Romulans considered this a future threat to their territory and in 2154, began to launch covert operations intended to destabilize that area of space. One attempt involved the growing Vulcan reunification movement with Romulus, which nearly re-ignited the conflict between Vulcan and Andoria. By 2156, however, there was no doubt of Romulus' involvement and war broke out.
The war ended in 2160 at the Battle of Cheron, in which the Romulans were soundly defeated. Following the battle, the two sides negotiated a treaty via subspace radio. Among other things, it established the Romulan Neutral Zone, entry into which by either side would constitute an act of war. Earth set up at least eight outpost stations on asteroids along its side of the Neutral Zone to monitor Romulan activity. This would be the extent of diplomatic ties with Romulus until the mid-24th Century.
(References: Enterprise- "In a Mirror, Darkly", "Babel One", "United"; The Original Series- "Balance of Terror")