The Jedi Order, later known as the Old Jedi Order and referred to as the Holy Order of the Jedi Knights, was an ancient monastic peacekeeping organization unified by its belief and observance of the Force, specifically the light side. The Jedi were the guardians of peace and justice in the Galactic Republic, and 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. Most often these threats were engineered by the Sith, the most prominent group of Force sensitives who wielded the dark side of the Force.
During several Great Schisms, numerous Jedi broke away from the original Order due to differences in how they believed the Force should be wielded. The various impasses reached by the orthodox Jedi and the defectors led to several wars, including the Hundred-Year Darkness, after which the defectors were exiled and founded the Sith Empire. The Sith were eventually found by the Republic and their discovery led to the Great Hyperspace War, and a Sith invasion of Coruscant. The Jedi on Coruscant, led by Jedi Guardian Anavus Svag and Jedi Master Memit Nadill, successfully defended the planet and defeated the Sith. 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 reformed and grew.
A thousand years later, after the Great Sith War, the Sith Empire manipulated the Mandalorians into attacking the indolent Republic. The Jedi Council forbade the Order's members from participating in the following war, but two Jedi, Revan and Alek, garnered the support of many of their peers and entered the war on the side of the Republic. Prevailing after a long and bloody conflict, Revan and the Jedi who had followed him disappeared after the war's final battle. When they returned, they had turned to the dark side, and during the war that followed most of the Jedi Order was either destroyed or seduced by Revan's teachings. During the aftermath the Order was left numbering less than one-hundred Jedi. In light of this, the remaining members of the Jedi Council officially disbanded the Order. Shortly thereafter, nearly all the remaining Jedi were consumed by Darth Nihilus when they convened on Katar. However, the exiled Jedi Knight Meetra Surik defeated the Sith Triumvirate of Darth's Nihilus, Sion, and Traya, and reformed the Order with the help of the Lost Jedi.
About three hundred years after the Jedi Civil War, the ancient Sith Empire had again become strong enough to directly challenge the Jedi and the Republic for control over the galaxy. The initial onslaught of the Sith Empire saw over half of the Republic's territory fall into Sith hands, but the Republic managed to check the Sith's advance in the Mid-Rim. After a time, the Sith Empire surprised the galaxy by offering a truce, which was accepted by the Republic. However, the Sith used the lull in the war to launch a surprise attack on Coruscant. The capture of the Republic capital allowed the Sith to extort many concessions from the Republic, and the resulting Treaty of Coruscant was slanted heavily in the Empire's favor. This began the Cold War, which lasted for about twenty years. Afterwards, the conflict began anew, and the Republic would eventually emerge victorious.
Centuries later, the galaxy was again engulfed in a war between the Republic and the Sith. Led by Lord Hoth, the Jedi were able to regroup and push back the Sith, who eventually withdrew to the planet Ruusan. After seven long battles on Ruusan, the Sith were defeated and presumed wiped out. One of them managed to survive and reorganize the Sith Order. Though Darth Bane and the Sith Lords that followed in his footsteps remained hidden, they were patient. Millennia later, they revealed themselves, determined to have their revenge.
At the height of a separatist crisis, the Republic was split in two and a war once again engulfed the galaxy. During the Clone Wars, the Republic's armies were once again led by the Jedi. During the war's first battle, nearly two hundred Jedi died before Master Yoda and a Clone army arrived to assist them. After nearly three 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 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 it's existence. Only a few survived the Great Jedi Purge. However, Anakin Skywalker, with the indirect help of his son, Jedi Knight Luke Skywalker, eventually defeated the Sith, and Luke formed the New Jedi Order during the time of the New Republic.