The Battle of Prantares was a conflict between the Earth and Romulan Star Empire.

It was data gathered from Operation Calypso which detected Romulan fleet movement to the Orion Colonies that allowed a Starfleet taskforce to successfully engage the Romulan forces. (FASA RPG module: Star Fleet Intelligence Manual: Agent's Orientation Sourcebook) It was known as the first real battle during the Romulan War where two squadrons of Romulan cruisers that were on their way to Rigel were met at Prantares by a Starfleet force of approximately equal size. The conflict ended in a tactical draw and whilst the Starfleet suffered heavy casualities, they succeeded in turning the Romulan forces back. (FASA RPG module: The Romulan War)

FASA RPG material states that the conflict was between the Federation and the Romulan Star Empire; but canon shows that during the Romulan War, there was no Federation. Presumably, the conflict was between the Romulans and either Starfleet, the Earth Alliance, Coalition of Planets or the United Earth.