The Tholian Assembly was the xenophobic interstellar government of the Tholian species. The territory of the Assembly included the region of space known as the Tholian sector. (TOS: "The Tholian Web", "Turnabout Intruder") This was an Alpha Quadrant power. (DS9: "For the Cause")
In 2293, the location of the Tholian Assembly in the Milky Way Galaxy was labeled in a star chart that was in Captain James T. Kirk's quarters aboard the USS Enterprise-A. (Star Trek VI: The Undiscovered Country, okudagram)
According to K'Ehleyr, in 2367, wars fought by the Klingon Empire tended to drag in the neighboring star systems, the Tholian Assembly, the Ferengi Alliance and eventually the Federation into the fighting. (TNG: "Reunion")
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 Earth vessel, 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 T. Riker used a notable strategy to calculate a sensory blind spot on a Tholian vessel. (TNG: "Peak Performance")
In 2372, one ambassador was acquainted with Benjamin Sisko of Deep Space 9, to whom he owed a favor. (DS9: "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. Odo considered this to be an impressive inroad into the Alpha Quadrant for the Dominion. (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 (p. 150), Tholia was in fact the home planet of the Tholians.
According to Star Trek: Star Charts (pp. 35 & 37, "United Federation of Planets II") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385), 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. The primary Tholia was a B-class star.
In the alternate reality comic book "The Tholian Webs, Part 2", Commander Loskene accuses the USS Enterprise of crimes against the Tholian Assembly, the ship having inadvertently intruded on Tholian space. Loskene plans to tow the craft back to the Tholian homeworld to answer for those deeds. However, once the Enterprise manages to escape, Loskene threatens the ship's crew with "the wrath of the Tholian Assembly" and warns that war will follow, the next time their civilizations meet.