Few singular moments had a more lasting impact on the Star Wars universe than Order 66. In the blink of an eye, the execution of Order 66 changed the entire galaxy, and the Jedi, forever.
While it was executed during Episode III: Revenge of the Sith at the direction of Darth Sidious, or Emperor Palpatine, the plan had actually been in place for some time. Count Dooku helped add Order 66 to the programming of the Clone Army, and as such laid the foundation for the execution of nearly all living Jedi and enemies of the Sith.
How did Order 66 happen?
For anyone who has seen Episode III: Revenge of the Sith, the aftermath of Darth Sidious directing the Clones to “execute Order 66” is one of the most heart-wrenching sequences in the prequels, and perhaps in all of Star Wars. After the word was sent, the Clone Army immediately classified all Jedi as traitors to the Republic who were to be executed immediately without question, and any who protected or stood by the Jedi were also in danger.
Due to Order 66, the clones began to systematically kill all of the living Jedi. Only a select few managed to escape the massacre, forming the foundation for the future of the galaxy and influencing everything that happened after Episode III: Revenge of the Sith.
This was a long-term plan by Darth Sidious, with the help of Count Dooku. There was initially programming in the clones to prevent them from turning on their commanders, but this was manipulated into an order to kill all Jedi, or those supporting or protecting them.
What was the result of Order 66?
When Darth Sidious chose to execute Order 66, it resulted in thousands of lives being lost. There were an estimated 10,000 or more Jedi prior to Order 66. After the Clones turned and began the slaughter, less than a hundred survived. In addition to the Clones, the newly turned Darth Vader took part in the execution of Order 66 by leading the raid on the Jedi Temple and killing masters, padawans, and even younglings.
Most of those who survived the initial attack were killed off in the next few years, leaving only a select few carrying on the legacy of the Jedi. Obi-Wan Kenobi and Jedi Master Yoda were the most significant survivors, carrying on their teachings years later to Luke Skywalker, and Luke later followed in their footsteps by teaching others like Ben Solo and Rey.
While the Jedi did ultimately survive in some form, the millennium that saw them spread across the galaxy with massive numbers was never seen again. The attack was so effective that many in the galaxy years later believed the Jedi to be a myth, or something that once existed but were entirely extinct.