The Jedi Purge was the systematic elimination of all Jedi from the Galactic Empire. The Purge began in BBY 19, when Supreme Chancellor Palpatine defeated an attempt by the Jedi Order to imprison or assassinate him and declared all Jedi to be enemies of the Galactic Republic. Pursuant to this declaration, he issued Order 66 to the Grand Army of the Republic and sent Darth Vader to lead a contingent of Clone troopers to the Jedi Temple, resulting in the deaths of most of the Jedi.
Palpatine quickly reorganized the Republic into an Empire, and declared himself Emperor. The Jedi continued to be public enemies, and those not killed in the initial purge were hunted by Darth Vader and the Imperial military for the duration of the Palpatine's reign.