SPOILERS FOR IATA:
Before the Empire existed, there was the Republic. They are one and the same. All of the systems which are part of the Empire were already united under the Republic. So what changed?
- The Republic was a peaceful place and peace was controlled and maintained by the Jedi who supported the Senate which made the laws.
- Someone set in motion the building of a Clone army to support the Republic. Given that the Republic was a peaceful place when taken as a whole under Jedi authority, this is obviously hidden for decades.
- Darth Sidious (Palpatine) began influencing and controlling the Trade Federation, leading them to separate from the Republic and rebel.
- Palpatine used the trade federation's rebellion to show a vote of no confidence in the current chancellor and take over in a 'surprise' vote.
- The Trade Federation unleashed a massive Droid Army built on Geonosis (where the Death Star was also being secretly built)
- Chancellor Palpatine is given more emergency powers to fight the rebellion which he swears he'll abandon when the war is over
- The Clone Army is unveiled. For 3 years, they fight alongside and under the command of the Jedis against Dooku and Grievous and the Separatist army.
- Palpatine continues gaining power by playing the two sides against each other.
- Palpatine uses Anakin to kill Dooku, then allows the war to end once he has his new apprentice.
- Palpatine uses Windu's attack to gain full and total control as the new Empire. The Republic is gone and the Empire rises up in its place. He has full and total control because the fear of the Clone Wars and the alleged Jedi takeover have created the climate for his rise.
It's no different than Hitler aside from the fact that Palpatine played both sides and that the Republic was a prosperous place before his rise to power. He used war to gain control and just turned all of the Republic's resources over to the Empire. It's not difficult to suppress an entire galaxy that joined you willingly.