Tarsonis was the capital world of the Terran Confederacy. A temperate planet, its original ten thousand inhabitants were from one of three prison ships to survive the journey from Earth to the Koprulu Sector. Tarsonis became the political and intellectual center of Terran power in the region as its population was the first to rediscover subwarp engines and colonize other worlds, forming the Confederacy.
By the beginning of the first game, the Confederacy had grown corrupt and proved inadequate at handling threats from the Sons of Korhal, the Protoss, and the Zerg. The defenses of Tarsonis were strong as the planet was protected by three orbital platforms, staging areas for the Confederate navy. However, the Sons of Korhal succeeded in disabling the central platform and planting a psi emitter at its location. The Zerg Swarm, lured by the emitter, descended upon Tarsonis by the billions and overwhelmed the planet, with further damage done as the Protoss arrived to combat them.
The Zerg still retained a presence on the planet at the time of the Brood War, and it served as a staging point for Kerrigan's quest to regain control of the Swarm. In StarCraft II: Wings of Liberty, Tarsonis is a broken and desolate world where the Dominion currently have several salvage operations.
Read more about this topic: Koprulu Sector