A war between Earth and the Romulans.
The Earth-Romulan War was fought between Earth and its allies and the Romulan Star Empire from 2156 to 2160. It came after the Romulans' attempts to destabilise the region, seen in Enterprise, failed to stop Earth, Vulcan, Andoria and Tellar from working together.
The war is known mainly from the original series episode Balance of Terror. Spock explains that it was fought with primitive atomic weapons, with no visual communication between ships, so neither side ever saw the face of the enemy. The war ended after the Battle of Cheron, an Earth victory.
The peace was negotiated by subspace radio and set up the Romulan Neutral Zone between the two sides. Because no one had met the Romulans in person, it was not until 2266, in Balance of Terror, that Starfleet learned they looked like Vulcans. The coalition formed in those years became the basis of the United Federation of Planets in 2161.