Order 66—the doom of the Jedi Order.
In the original storyline, around a year later in 19 BBY, the Galactic Republic was winning victory after victory in the war, and the Separatists' end was near. Chancellor Sheev Palpatine's control over power had also reached new heights.
The Jedi Order therefore grew increasingly wary of Palpatine. But Palpatine had long since completed his preparations and no longer concealed his identity. After learning that Palpatine was the Sith Lord Darth Sidious, Master Windu led three Jedi Council Masters—Kit Fisto, Saesee Tiin, and Agen Kolar—to arrest Darth Sidious.
However, Darth Sidious's power at that moment far exceeded all their expectations. The three Council Masters were killed almost instantly, leaving only Master Windu to fight him desperately.
Upon receiving the news, Anakin Skywalker rushed to the Republic Senate Building, only to see Darth Sidious pretending that Master Windu was about to kill him. Anakin had already been gradually drawn toward the Dark Side because of his mother's death and his distrust of the Jedi Order. Now, believing Darth Sidious could help him resurrect his mother, he intervened.
But consumed by rage and fear, Anakin lost his reason and severed both of Master Windu's hands with a single slash. Darth Sidious then blasted Master Windu from the Republic Senate Building with Force lightning.
Thus, Anakin Skywalker completely fell to the Dark Side and became Darth Sidious's disciple. Having finally found leverage against the Jedi Order, Sheev Palpatine used their alleged attempt to assassinate the Chancellor as an excuse to declare the Jedi Order guilty of treason and activate Order 66.
Order 66 was one of 150 emergency orders implanted into Clone Troopers' brains through biochips. These orders covered all kinds of extreme situations; for instance, Order 67 dealt with a Chancellor committing treason. Order 66,