Star Wars Episode III: Revenge of the Sith
Palpatine sends Vader to the Jedi Temple to murder all of the Jedi inside. Across the galaxy, all of the Republic's clone soldiers suddenly betray their Jedi Generals and kill them. This is because the clone troopers are actually under the control of Palpatine. Within minutes, all of the Jedi are dead, except for Yoda and Obi-Wan.