I have a theory that there was no united empire under the Sith. At the time there were many Sith and Sith factions. Each faction controlled part of the galaxy and they were constantly warring with each other, with each Sith Master wanting to be the guy who gained control over everything, but not only did they have to contend with each other, they had to watch their backs because their subordinates were always waiting to strike. All of this would have been seen as healthy because it would ensure that the strongest Sith always led their territories. There may have a single Sith Council made up of the leaders of the Sith factions who may have met to discuss matters that concerned all of them, such as the Jedi but for the most part they were enemies. I also think the Sith would probably have had slave armies and that their method of rule (call it the Rule of Strongest) with each Sith territory being a separate entity, is what would have allowed the Jedi to gain a foothold. The Jedi couldn't take on an entire galaxy. But take one world, one local Sith ruler. Depose him and nobody cares. The other Sith think it's just a territorial conflict with a rival Lord. The slave army is freed and those slaves are asked if they want to join the Jedi army and free the galaxy. Some choose to join, others choose to go back to their families. This happens on another worlds. And another. And another. Which each freed world, more freed slaves choose to join the Jedi army and the Sith are too busy fighting each other to pay much attention because they never conceived that the peaceful Jedi would ever go to war. By the time the Sith realise what's going on, the Jedi have gained a solid position in the galaxy. What's more, rumours of the Jedi begin to spread to slaves across the galaxy. Slaves who spent their entire lives living in fear of the Sith are given hope for the first time and as the war goes on, that hope becomes courage as slaves within the Sith territories begin to rebel. And even with all of this going on, the Sith still can't stop fighting each other. Logic dictates they should combine their strength but they're all so consumed with wanting to be sole rulers of the galaxy that it blinds them to what they should do and leads to their downfall. At some point, the Sith can no longer trust or maintain their slave armies due to constant rebellion and are forced to turn to droids. But by then their defeat is inevitable. They're facing war on three fronts - a Jedi army that grows with each liberated world, a slave rebellion and of course, each other. I think that a Sith Lord, realising the certainty of defeat, takes drastic action. This Sith Lord is a Darth Bane kind of character and before the end of the war, he enacts a plan to destroy all the rival Sith Masters, Lords and apprentices. This would have to happen before the end of the war, probably a few years before, because the Jedi in TPM are aware of the Rule Of Two. The remainder of the war would see one master and one apprentice in control of a droid army and even though they know they're going to lose, they keep fighting because to only way to enact their plan for revenge is to make the Jedi think they've been wiped out completely. By the time the Jedi army reaches Coruscant to fight the final battle of the war, the Sith are gone, presumed dead. The galaxy is now free but there's lots of rebuilding to be done. Millions of freed slaves unite and work together to build a better galaxy, ultimately forming a new Galactic Republic and the Jedi, who prepare to return to their temples and monasteries, are asked to stay and protect the young Republic as guardians of peace and justice. Meanwhile, somewhere in the galaxy, two Sith, a master and an apprentice, begin their thousand year plan to enact terrible vengeance on the Jedi and rule the galaxy once more.... Now, what I've just written is basically a concept I've had for a number of years for a project called STAR WARS: Prologue, which would tell the story of the old Sith conflict and the origins of the Republic, separate to the expanded universe and based on what we learn in the movies and the little information Lucas has given over the years plus a lot of my own ideas.