A long long time ago...you know the rest
The Great Sith War
“The Dark Lords of the Sith have risen again. Together, we will bring down the galaxy.”
―Exar Kun t
The Great Sith War, also known as the Sith War, the First Sith War, and the Exar Kun War, or the Great Shadow Crusade to the Mandalorians, was a galaxy-wide conflict which took place in 3996 BBY. One of the largest engagements of the Old Sith Wars, the Great Sith War saw the rise of the Sith, once again in opposition to the Jedi Order. The roots of the conflict are traced back to the fall of Freedon Nadd from the Jedi way and his ascension to Dark Lord of the Sith. Under the combined efforts of the Jedi, the Republic forces were able to drive the Sith forces out of Republic space. However the combined losses of both the Jedi and Republic forces meant they were not able to pursue the Sith survivors and thus the Sith survived.