Far from Earth, in a region of space called the Koprulu sector, the terran-colonized world of Mar Sara was at a critical juncture. The alien zerg, a hive-mind bent on devouring all life, had infested the planet, and the Confederacy (the government overseeing Mar Sara and other terran worlds) had evacuated its military personnel, abandoning the colony to be torn apart by the swarm. Marshal Jim Raynor joined forces with a rebel group called the Sons of Korhal, led by ambitious, charming Arcturus Mengsk, to battle the aliens and rescue civilians. Though the rebels were able to save lives, the planet was consumed by the zerg, then purified from orbit by the protoss (an advanced civilization bound together by a communal psychic connection called the Khala).
With Mar Sara fallen, the Sons of Korhal (including Raynor, Mengsk and Sarah Kerrigan, who had trained as a Ghost, a psionic hitwoman for the Confederate government) regrouped on the planet of Antiga Prime. Jim and Sarah bonded quickly; the straight-talking marshal and the telepathic assassin made an effective pair. When the zerg invaded Antiga Prime, Mengsk suggested that the Sons utilize Confederate psionic projection technology – psi-emitters – to broadcast signals that would goad the aliens into attacking the Confederacy. His tactic proved ruthlessly efficient.
Flush with success, the Sons besieged Tarsonis, the capital of the Confederacy, with psionically-directed zerg. Sarah and Jim expressed reservations about using the zerg as a weapon, but Mengsk was resolute. The concentration of zerg drew a protoss assault, and Mengsk dispatched Kerrigan with a small force to fend them off, enabling the Sons to sack the planet.
Unbeknownst to Sarah, she had assassinated Arcturus’s father while serving as a Ghost – and her decision to question orders made it clear that her usefulness had ended. As Tarsonis crumbled, Mengsk abandoned Kerrigan to the zerg.
Raynor and his crew left the Sons in disgust. Though they later stole Mengsk’s flagship, the Hyperion, Arcturus was unmoved. He reconvened the destroyed Confederate government as the Terran Dominion, and an old tyranny with a new face was born in the Koprulu sector.