The Romulan Star Empire was a state which bordered the Federation and Klingon Empire. Little was known of the origin of the Romulan race, except that they were an aggressive offshoot of the Federation's Vulcan culture. The Romulans fought an inconclusive but exceedingly fierce war against the Federation a century ago. A Neutral Zone agreed by treaty separated the two societies. Neither race was permitted to enter this Neutral zone, but violations on both side occurred.
Romulans were of the same genetic stock as Vulcans, but did not believe in the Vulcan values of peace and logic. Their ships usually fought to the death; they rarely took prisoners, and if defeated, they would self-destruct instead of surrender.