The Tholian Assembly was the xenophobic government of the Tholian species. The territory of the Assembly included their homeworld and its colonies, at least some of which were located in the Tholian sector. The Assembly shared borders with the Federation and the Ferengi Alliance. Though it didn't share a border with the Klingon Empire, the Assembly could be influenced by events in the Empire. The space between these two galactic powers was populated by non-aligned systems. (TOS: "The Tholian Web", "Turnabout Intruder"; TNG: "Reunion"; DS9: "For the Cause", "Call to Arms") The location of the Tholian Assembly in the Milky Way Galaxy was depicted on a Federation star chart in 2293. (Star Trek VI: The Undiscovered Country, production art)
By 2152, the Vulcan High Command had only limited contact with the Assembly. When Tholian ships traveled far beyond their territory to salvage a 31st century time-travel pod, it was discovered that the Assembly may have been a faction in the Temporal Cold War. (ENT: "Future Tense")
In 2268, the USS Defiant entered a region of space annexed by the Tholian Assembly in response to a fake distress call sent by mirror universe Tholians through an interphase. Three weeks later, the starship USS Enterprise attempted unsuccessfully to rescue the Defiant from the spatial interphase. (TOS: "The Tholian Web"; ENT: "In a Mirror, Darkly")
The 2350s saw an increase in hostilities between the Assembly and the Federation. In 2353, the Tholians destroyed a Federation starbase. (TNG: "The Icarus Factor") Starfleet developed Tholian-based battle simulations. In 2355, during one such simulation, William Riker used a notable strategy to calculate a sensory blind spot on a Tholian vessel. (TNG: "Peak Performance")
In 2371-2372, a Tholian ambassador was present on Deep Space 9. (DS9: "Defiant", "Life Support", "The Way of the Warrior") In 2372, a Tholian observer was killed in a bombing on Earth by a Dominion infiltrator. (DS9: "Homefront") In 2373, the Tholian Assembly signed a nonaggression pact with the Dominion, and remained neutral during the Dominion War. (DS9: "Call to Arms") In 2379, there was a Tholian ambassador on Romulus. (Star Trek Nemesis)
"Tholia" was a word - possibly a proper noun - that Klaang mentioned in the Star Trek: Enterprise episode "Broken Bow", but which Hoshi Sato could not translate. According to The Worlds of the Federation (page 150), Tholia was in fact the home planet of the Tholians.
"The Explored Galaxy"
This chart was also seen in several Star Trek: The Next Generation and Star Trek: Deep Space Nine episodes set in the 24th century, from the year 2364 to 2370. These were: TNG: "Conspiracy", "The Measure Of A Man", "The Emissary", "The Mind's Eye", "The Game" and DS9: "In the Hands of the Prophets", "Cardassians".
Tholia, the capital world of the Tholian Assembly, was located in the Alpha Quadrant. Federation scientists theorized that Tholia was a Y class planet. The dominant species on Tholia were the non-humanoid Tholians. Diplomatic relations were established with the Federation in 2271. Tholia orbited a B-class star with a magnitude of -7. (Pgs. 35, 37)