More than three thousand years before the rise of Darth Vader, the galaxy far, far away was divided by a massive war between the Galactic Republic and a vast and powerful Sith Empire. Raging on for decades, this Great Galactic War only came to a halt when the Sith Lord Darth Malgus led a surprise attack on the Republic’s capital world, Coruscant. His devastating attack forced the Republic to sign the Treaty of Coruscant, bringing about a period of uneasy peace between these rival factions – and that’s where the storylines of Star Wars: The Old Republic begin.

In the following years, tensions between the Republic and Empire only grew. Despite the Treaty of Coruscant, both factions took every opportunity to undermine the other. The Empire annexed many defenseless worlds and encouraged others to openly reject their affiliation with the Republic, while the Republic resisted their ancient enemies’ aggressions as best they could. Jedi and Sith, spies and soldiers, even hired guns–all played their parts in the escalating conflict.

After twelve years of tenuous peace, all-out war resumed between the Republic and the Empire. When the Sith Emperor Vitiate was seemingly destroyed by a Jedi, many hoped it would bring a swift end to the hostilities, for good. Instead, various splinter factions, hoping to fill the vacuum of power, multiplied the number of battlefronts.

Darth Malgus–long dissatisfied with the political maneuvering and backstabbing of Imperial leadership–led one of these fractured groups in a revolt, hoping to seize control of the Sith Empire. Malgus and his followers were challenged by the Republic and the Empire both, and he was presumed dead when his stolen space station was destroyed. But the damage he inflicted left the Republic at a considerable advantage, signaling a shift in the renewed war.

Story above copied from a 2021 forum post by Charles Boyd, transcribed from a video on that post. It can be found here.