The Old Jedi Order, also known as the Order of the Jedi knights, and later referred to as the Old Order, was an ancient monastic peacekeeping organization unified by its belief and observance of the Light Side of the Force. The Jedi were known throughout the Galaxy as the guardians of peace and justice in the Old Galactic Republic, having existed for over a thousand generations. They became perhaps the most well known of the various groups that relied on the Force for their power.
Led by a series of Jedi Councils, the Order grew over the millennia despite facing many trials, eventually becoming prominent throughout the Clone Wars. Most often these threats were engineered by their antithesis, the Sith Order, the most prominent group of Force sensitives who wielded the dark side of the Force.
Many members of the Old Jedi Order, and thus the Sith Order, were direct descendants of members of the ancient Je'daii, the earliest known Force-sensitive Order in Galactic history. Over the millennia, numerous Jedi broke away from the original Order due to differences in how they believed the Force should be wielded. Consuming themselves with the dark side of the force, the Sith Order was founded, and after several wars, including the Hundred-Year Darkness, the defeated Sith were exiled, eventually finding the planet of Korriban.
The Jedi were once again forced to face the Sith when the Sith Empire was discovered in the Outer Rim Territories, defeating them During the Great Battle of Coruscant, where the dominance of the Galaxy was given to the Republic. Again and again however, the Sith would be reborn from within the ranks of the Jedi Order, and many more wars would be fought. Additionally, the ancient Sith Empire had not been as completely destroyed as the Jedi believed, and over the next thousand years, it managed to reform and gain power in the Outer Rim once again. However, it was destroyed when the Republic and the Mandalorian Empire joined forces, causing the survivors to flee to Ruusan, another Sith holdfast. After seven long and devastating battles on Ruusan, the Sith were defeated and presumed wiped out. One of them however, managed to survive and flee to Korriban.
Now free of any possible threat by the Sith, the Old Jedi Order remained as the guardians of peace in the Republic, strictly acting under the command of the Galactic Senate. The most prominent symbol of the Jedi at this time was their Grand Temple, which was erected on Coruscant, the capital of the Republic. Believing the Sith were gone, they become mostly a secretive order, allowing non force-sensitive individuals to run the Republic, only aiding them in times of crisis. On millions of planets, the Jedi were often called a myth, or simply a cult of crazed fanatics and wizards.
In 35 BBY, the galaxy was beset by political instability and social unrest, resulting in a secessionist crisis that ultimately led to the formation of the Galactic Confederacy. Using legions of clone armies, the Confederacy began large scale invasions and battles against the Republic, and the Jedi Order was forced to help lead the Republican armies into battle, under the command of Jedi Grand Master Yoda. After almost 15 years, the course of the war swung in the Republic's favor, and total victory for the Jedi and the Republic seemed in sight.
However, the war itself was simply the penultimate stroke of the Sith Grand Plan, orchestrated by the Supreme Chancellor of the Republic, Palpatine, who was in fact a Sith Master. The Jedi eventually learned the truth, but could not stop the plan's execution; Palpatine turned Anakin Skywalker to the dark side, who became the new Sith Lord Darth Vader, and instigated the Great Jedi Purge. The Order was virtually annihilated by the Sith and betrayed by the Republic they had served for nearly all of its existence. Many would survive the purge, but would later be killed off through the combined efforts of Vader and the Inquisitors. In spite of the Sith's efforts, however, the Jedi continued to endure.
With the Old Order virtually extinct, Palptine and the Sith gained near control over the Galaxy, far greater than the Ancient Sith Empire had. However, within a generation of the Empire's rise to power, Luke Skywalker—the son of Vader—was trained in the Jedi arts by Obi-Wan Kenobi and Yoda, both of whom survived the initial purge of the Order. Redeemed by his son's compassion, Anakin Skywalker returned to the light in 4 ABY, destroying both Vader and Palpatine during the Battle of Endor.
With the end of the Galactic Civil War, the fall of the Empire, and the rise of the New Republic, Luke Skywalker ultimately became the Jedi Master to a new generation of Jedi—fulfilling the promise he made to Yoda to pass on what he had learned. However, in these initial years, his promising recruits had died at the hands of the Imperial Remnant and Palptine, who had survived through the use of various clone bodies he had in place to survive. After defeating Palpatine once and for all, Luke would participate in one last decisive battle against the Imperial Remnant under the command of Grand Admiral Thrawn, and after Thrawn's defeat and death, Luke officially founded a new Jedi Order.