The Galactic Empire is one of the main factions in the fictional universe of Star Wars. It is a galaxy-spanning regime established by the series' lead villain, Palpatine, to replace the Galactic Republic in Star Wars Episode III: Revenge of the Sith. The Galactic Empire is introduced in Star Wars Episode IV: A New Hope. The Empire also appears in Star Wars Episode V: The Empire Strikes Back, and in Star Wars Episode VI: Return of the Jedi.
The Empire's origins are explained in Star Wars Episode II: Attack of the Clones and Star Wars Episode III: Revenge of the Sith, where it replaces the Galactic Republic in the midst of a galactic civil war orchestrated by Palpatine, then the Republic's Supreme Chancellor. In a scene towards the end of the film, Palpatine appoints himself Emperor in the presence of the Galactic Senate, afterward the Imperial Senate. By the time of A New Hope, the Empire has transformed into a merciless totalitarian regime, still struggling with the Rebel Alliance.
The Galactic Empire is born out of the collapsing Old Republic. However, the seeds of change are planted during the Clone Wars, the epic war between the Republic and the Confederacy of Independent Systems depicted in Attack of the Clones.
When the extent of the Separatist threat becomes clear in Episode II, the Old Republic Senate, the legislature of the Republic, grants Palpatine extended governmental powers to deal with the crisis. This conflict allows Palpatine to remain in office long after his term as Chancellor officially expires. Palpatine promises to return his powers to the Senate once peace and order is restored to the galaxy. His first order is to create an army of clone trooper. He takes advantage of the conflict to increase the office's power until by Episode III, he is effectively a dictator.
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Official Site: Lucasfilm
Links: Wikipedia Galactic Empire Databank Star Wars.com