Setting: Medieval "Europe". Typical technology, swords, armor, bows, catapults, cannons, all that good stuff.
Magic: The ability to control the elements. Similar to the popular show Avatar, some people are born with the ability to control the elements; water, fire, earth, air. The magical being controls the element by manipulating the energy within it.
Background: After a long war between these benders and non magic folk the benders lose. Instead of exiling them, the winning side decides to use the benders to their advantage. The agreement made at the end of the war was that the benders now belonged to their enemy government, which they would be used to advance technology and power. Examples: Used in mines, on boats, forging, mills, etc.
The Problem: Of course there would be a resistance to this basically slavery life. However my story takes place many, many years and generations after the initial end of the war. These benders are still lower class and used to power the government. How would generations later of benders still be able to be controlled?
Question: How could a government of non magic users control these benders?
What I'm Looking For: Ideas of possible control mechanisms and/or how governments have controlled their people in history. Of course any other ideas are welcomed.