The Tellun star system included two inhabited planets, Elas and Troyius, each bearing humanoid civilizations. This system was located in the Beta Quadrant.
A period of Elasian-Troyian interplanetary war intersected with the Federation-Klingon Cold War in the 23rd century. By 2268, both the Federation and the Klingon Empire considered the Tellun system to reside within their respective borders. The Federation sought an end to the internal conflict and served as a mediator between Elasians and Troyians, while Klingons tried to subvert the peace efforts.
In 2268 a secret diplomatic mission of the USS Enterprise to facilitate the marriage of Elaan, the Dohlman of Elas, to the ruler of Troyius, was interrupted by sabotage and an indecisive ship-to-ship engagement with a Klingon D7 class battle cruiser. During the course of battle, the Klingons' true interest in the Tellun system, access to Elasian dilithium deposits, was discovered. (TOS: "Elaan of Troyius")