While the Sith remained hidden for centuries, they would not do so forever. With the benefit of hindsight, their influence in the events leading up to their rise to power is clear. The Sith had waited two thousand years and knew that patience was the key to executing their master plan. Acknowledging that open warfare with the Jedi and the Republic had proved to be unsuccessful in the past, the new Sith's plan avoided any open confrontation with the Jedi and was executed with two goals in mind. The first would be to eat away at the Jedi's ranks, gradually reducing their numbers. The second was to insert one of their own into a position of power within the Republic government to eat away at it from within.
It is unknown just exactly when the Sith's plans were set into motion, but history would give credit to the Sith Lord, Darth Sidious. In the years leading up to the Sith's domination, the rate of planetary and sector conflicts increased and claimed the lives of the many Jedi charged to deal with them. Also, the Sith gradually worked their way up the political step-ladder and eventually took control of the Republic Senate. All of this unbeknownst to the Jedi and the citizens of the galaxy. The Sith had had two thousand years to perfect the art of secrecy. Virtually no one had any suspicions or proof of the Sith and their involvment.
From his position, Darth Sidious acted as puppetmaster, orchestrating a war that would engulf the galaxy. Sidious instigated a separtist movement that would split the Republic in two and culminate in the galazy-wide conflagaration known as the Clone Wars. Rather than fight the Jedi and the Republic openly, Sidious would have the two destroy themselves from within. Many Jedi went to their deaths believing that their fight was to hold the Republic together. In reality, they were all pawns in the Sith Lord's schemes. It was during the Clone Wars that Sidious discovered his final and most powerful apprentice, Darth Vader. With the two Sith Lords working together, the Jedi and the Republic did not stand a chance. Out of the ashes of the Clone Wars, Darth Sidious declared himself Emperor and instituted an Empire based on fear and obedience.
The Jedi had lost, and seemingly the Sith had won. However, the citizens of the galaxy banded together to fight against this oppresive Empire and formed a movement known as the Rebellion. Starting out as pockets of resistance throughout the galaxy, these rebels banded together to form an alliance. But in the face of the mighty Empire, the prospect of victory seemed hopeless. However, a new hope emerged in the form of young Luke Skywalker, Jedi legacy and strong in the Force himself. Luke would join in the fight against the Empire which would end with a showdown between Skywalker, Darth Vader, and the Emperor himself.