A Tzenkethi-held world towards which a Changeling saboteur steers the cloaked and fully armed Defiant in The Adversary, intending an unprovoked attack that would plunge the Federation into war.
M'kemas III is a planet in Tzenkethi territory with a settlement on its surface.
In The Adversary, the Changeling who has been posing as Ambassador Krajensky seizes control of the Defiant's helm, weapons and cloak and locks the ship on a course for M'kemas III. If the vessel arrived it would open fire on the settlement automatically, and the Tzenkethi would have every reason to declare war on the Federation.
Unable to regain control in time, Sisko arms the auto-destruct rather than let the attack happen. Odo's fight with the Changeling in engineering allows the crew to recover the ship with moments to spare, and the Defiant turns back before reaching the planet.