The quarantine zone was an area of space located in the galaxy's Sagittarius Arm, centering on the star Dakiak.
In the year 2268, Starfleet Command established the quarantine zone after twenty Federation starships mysteriously switched their allegiance to the Klingon Empire upon entering the region. After learning that the size of the region was expanding at an accelerating rate and would envelop the entire Federation within months, Starfleet Command assigned the USS Enterprise to enter the zone and reverse its effects, then isolated the zone completely by enclosing it within a Klein sphere. (TOS video game: The Rebel Universe)