Darth Sidious enacted his fateful Oder 66 and wiped out virtually every Jedi in the galaxy. But before the dark times, a small window of opportunity existed for a few. There were times when someone could have stopped him and ended his war.
The Jedi council is top tier when it comes to a group having the ability to stop Darth Sidious and even defeat him. Going as far back as even decades before the events of the Phantom Menace, a time when Sidious was but an apprentice to Darth Plagueis.
Notably, Qui-Gon Jinn suspected something was up before episode one. Quigon’s doubts were affirmed when he encountered Darth Maul on Tatooine. The evidence of a sith warrior began to open the eyes of the council. Qui-Gon was killed by Maul before he could follow the trail back to Palpatine/Sidious.
Count Dooku of Serreno, upon finding out Palpatine was a Sith Lord known as Sidious, had an opportunity to bring this incriminating evidence to the council, but instead chose to team up with Sidious. If you can’t beat ’em, join ’em! Dooku, a decade later would make an offer to Kenobi to pair up, “You must join me, Obi-Wan, and together we will destroy the Sith!” Kenobi and the council did not trust Dooku and still had doubts as to what truth, if any, existed of this sith lord.
Master Yoda could have defeated Darth Sidious if he would have been truly prepared to face the evil Sith. If the battlefield would have been different or if Yoda and Windu would have teamed up to take down the Sith, the Jedi and Republic would have been saved.
Anakin Skywalker could have thwarted Darth Sidious and in fact did, albeit almost 25 years after he was given the initial opportunity. Anakin fell into Sidious’ trap and attacked Windu, as Sidious played victim, only to then spring to life and fire lightning upon Windu. Nearly 25 years passed. Aboard the second Death Star, Anakin/Vader watched as his son Luke is struck with lightning by Darth Sidious. “No!” The time had come. He lifted his master, absorbing the deadly lightning, and tossed him over the reactor.
There were several people who could have in fact played a key role in defeating this Sith lord Darth Sidious, but were either too late, just made the wrong choice or simply did not believe in the return of the sith. A true shame when you stop to think of how many lives this Sidious was responsible for ending in the Star Wars universe.