The Earth-Romulan War setup the Neutral Zone
The Earth-Romulan War was a major interstellar conflict fought during the mid-22nd century between the forces of United Earth and those of the Romulan Star Empire, and their respective allies.
The two sides knew little of each other. Starfleet prior to the 23rd Century had never seen a Romulan visually. However, before the war, the Romulans began a campaign to sow distrust between the major political entities, including those of Earth, Vulcan, Andoria, and Tellar.
This campaign included impersonating ships to increase tensions, especially between the Vulcans, Andorians, and Tellarites. Only Earth kept the fragile group united.
The exact start and end dates for the war are a mystery. With the two major sides never physically meeting each other, it seems the conflict was almost entirely space-faring.
Spock categorized the weaponry generally used in the Earth-Romulan War as atomic in nature, however the Enterprise series casts doubt on this prospect. However, it is entirely possible that atomic weapons were used, perhaps as mines or booby traps.
Romulan starships were noticeable by the "bird of prey" paint scheme.
The Battle of Cheron is generally considered the decisive battle in the Earth-Romulan War.
Following the battle, the two sides negotiated a treaty via subspace radio. Among other things, it established a Neutral Zone, entry into which by either side would constitute an act of war. Earth eventually set up at least eight outpost stations on asteroids along its side of the Neutral Zone to monitor Romulan activity.
The war is widely credited as leading to the formation of the United Federation of Planets.