An excerpt from Memory Alpha:
The Earth-Romulan War, also known as the Romulan Wars, was a major interstellar conflict fought from 2156 to 2160 between the forces of United Earth and those of the Romulan Star Empire. Earth was assisted by its allies Vulcan, Tellar, and the Andorian Empire. (ENT: "In a Mirror, Darkly, Part II")
The two sides knew little of each other before the war, and little after the war. Based on the limited state of knowledge that still existed in as late as 2266, the war was summarized as "Earth believes the Romulans to be warlike, cruel, treacherous; and only the Romulans know what they think of Earth."
Events depicted in Star Trek: Enterprise's fourth season leave open the possibility that the Romulans may well have had greater knowledge of Humans than Humans did of them. Certainly their plan to turn the various races of the region against one another would require a fair amount of intelligence on each of them.
Among the casualties during the course of the war include several members of the Stiles family, including one Captain Stiles. (TOS: "Balance of Terror")
The Battle of Cheron is generally considered the decisive battle in the Earth-Romulan War. Both sides recognized the major victory achieved here by Earth and its allies as a humiliating defeat of the Romulan military. This event would establish long-term political effects within the Romulan Empire lasting well into the 24th century. (ENT: "In a Mirror, Darkly, Part II";TNG: "The Defector")
Following the Battle of Cheron 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 set up at least eight outpost stations on asteroids along its side of the Neutral Zone to monitor Romulan activity.
For several years it was believed that the Treaty of Algeron was the designation for the treaty resolving this war, but this was later disproved in TNG: "The Pegasus".
From a 23rd century Human perspective, the Neutral Zone represented an area "between planets Romulus and Remus and the rest of the galaxy..." Some Humans still harbored resentment over the war after over a century of peace, including Lt. Stiles of the USS Enterprise.
Despite these Human attitudes, until 2266 the Neutral Zone served its purpose and the treaty had remained unbroken. In fact, one might say it served its purpose too well, as there were limited or no contacts between Earth and the Romulan Star Empire from the conclusion of the war until the mid-23rd century. (TOS: "Balance of Terror")