An interstellar power neighbouring the Federation that fought a war against it in the mid 24th century and was nearly drawn into another by a Changeling saboteur aboard the Defiant.
The Tzenkethi are never seen on screen, but their state looms over the Deep Space Nine episode The Adversary. Benjamin Sisko, newly promoted to captain, recalls serving under Leyton aboard the USS Okinawa during the last Federation war with them.
In 2371 Ambassador Krajensky brings word of a coup on the Tzenkethi homeworld and orders the Defiant to the border to show the flag. Krajensky is in fact a Changeling, who sabotages the ship and sends it under cloak towards a Tzenkethi colony with weapons armed, intending to start a new war that would weaken the Alpha Quadrant.
The crew regains control only after Odo kills the infiltrator, the first time a Changeling has harmed another. The earlier war is mentioned again in Paradise Lost.