[MuGen]

It wasn't the Empire... it was the Galactic Republic. The Emperor or.. Sideous created a plot that incriminated the JEDI for trying to overthrow the Republic, namely the Supreme Senator. If you heard once Anakin turned, he holo'd every Clone and said,

"Every single Jedi is now... an Enemy of the Republic"

And as the senate went into hiding, Sideous gained supreme power over the Republic, making it a Imperialist kingdom, thus Empire.