One too many wars, one too many natural disasters, one too many instances of chaos and destruction caused the Division. A group, known as the Subjects, were taken into "protection", a place where they would be safe from the destruction that came before them while everyone else was left on the Outside, forced to deal with their conditions or die.
If only things were that clean cut. It's impossible to know the truth when one side is spoon feeding you all the information. It takes one on both sides to decide what the truth actually is.