Use this question as an opportunity to strut your Engels (see 101.1). Start by giving us a little context. Who is the author? When is he writing? In what sense is his an historical or developmental story? What argument is this idea a part of? Where does Engels think the class antagonisms come from?
Most societies have a higher class exploiting an oppressed class, this is how states usually exist. So, states arose out of a necessity to maintain and control the clashes between these classes.
See also: Q375.