The war began when the First Sith Empire attacked the Galactic Republic. Sith forces assaulted Coruscant while Naga Sadow directed the battle from a meditation sphere orbiting a giant star. His apprentice betrayed him, attacking the meditation sphere. The apprentice was defeated, but Naga Sadow's battle meditation was interrupted and the Sith forces on Coruscant were subsequently overrun. The Republic fleet pursued the Sith forces back to Naga Sadow's meditation sphere. There, the Dark Lord caused the star to explode allowing him to flee to Korriban.
Upon his return, Naga Sadow discovered that Civil War had begun in opposition to his rule. He was able to crush the revolutionaries but his military was in ruins. Soon after, the Republic attacked Korriban completing the destruction of the Sith armies. Naga Sadow fled yet again into exile on Yavin 4. The Republic and Jedi Order proceeded to exterminate all remnants of the scattered Sith Empire. The remaining Sith, along with a new Emperor, fled the empire into deep space.
They wandered for 20 years before stumbling on the jungle planet of Dromund Kaas where the new Sith Empire's capital was founded.