Spoiler Alert: Details from Episode III in this section
The Jedi Council was opposed to Emperor Palpatine (Darth Sidious). The Emperor named the Jedi enemies of the Republic and used one of the Clone Wars' greatest heroes, a Jedi named Anakin Skywalker, to lead the clone army against the Jedi. Anakin killed Darth Tyrannus, went over to the dark side, and became Darth Vader, the most legendary of all Sith Lords.
Vader and his clone army began to hunt down and destroy all of the Jedi. Vader eventually faced his former Jedi master, Obi-Wan Kenobi, in a lightsaber battle. Kenobi defeated him and Vader was seriously wounded. He was restored by several operations that made him half man, half machine. Now in his new form, Darth Vader became the dark enforcer of the Emperor. Sidious dissolved the Republic, founded the Empire, and for the first time ever, secured the Sith as the rulers of the galaxy.