The Jedi Knights of the Old Republic truly were guardians of peace and justice throughout the galaxy. The Old Republic, already established for thousands of years, strained their resources to the limit holding together the intragalactic bonds built up over millennia, while continuing to explore uncharted regions of space to find and incorporate more and more civilizations into the galactic community. The galaxy was, by and large, still a wild and dangerous place, and the Jedi Knights were called upon time and time again to answer the threat of conflicts that would weaken the foundations upon which the Old Republic was built.
It was during this time period that the Jedi Knights first contested with their sworn enemies, the Sith, who they would do battle with on numerous occasions for over five thousand years. The battles the Sith waged against the Jedi proved to be some of the bloodiest campaigns in the history of the Jedi, engulfing the entire galaxy in all out war. Pitting brother against brother, student against master, the Jedi Order was strained to its limits during these dark times.
The Jedi's first contact with the Sith resulted in what history has labeled the Great Hyperspace War. A millennium later, the Sith threat reappeared in the form of two fallen Jedi, Exar Kun and Ulic Qel-Droma, who's war literally destroyed planets and star systems. At the conclusion of that conflict, the galaxy was only afforded a decade of peace until a Sith resurgence once again called the Jedi into battle and plunged the galaxy into warfare. The Jedi once again emerged victorious and thought the Sith defeated once and for all.
However, followers of the Sith tradition remained and new Sith lords arose. For two thousand years the Sith regained their strength and replenished their forces. United by the charismatic Dark Lord, Kaan, the Sith united under a Brotherhood of Darkness and openly engaged the Jedi. This war proved to be the war that would ultimately determine the fate of the Sith and usher in a golden age for the Jedi Knights.